Implement customizable formatting

[ngehrsitz]

This PR adds the functionality to customize how imports are formatted by introducing the concept of sections.
Sections are specified at run time and describe the desired output format of the imports. Every import is parsed and assigned to the section that it matches best with. Some sections are used purely for formatting purposes like NewLine and never match any imports. Some sections may also have a prefix or suffix that is only rendered if there were imports matched to the section.
The CLI is now built upon cobra and introduces the diff,print and write subcommands with support for the new functionality.
Backwards compatibility to the old CLI is kept with the only exception being that -local needs to be specified as --local now.
The corresponding changes to golangci-lint are also ready and I will open a PR there once this has been merged.
Test cases based on #39 were also added.

[daixiang0]

Could we merge print and write together as write seems more like an option for print, how about gci format?

[ngehrsitz]

The reason for choosing write and print is that I think they make it immediately clear to the user what they do. With format I would not be sure if it reformats the file or just prints a formatted version.
However I could easily add an alias like there is with overwrite for write.

[daixiang0]

In fact print may mislead as well, let's make it clear and easy for users:

• gci <files> to print formatted version
• gci -w <files> to write formatted version

Keep same logic with goimports makes users easily migrate, what do you think?

[ngehrsitz]

The "old" style were you can simply do gci <files> and gci -w <files> is still fully present and functional to provide backwards compatibility. Thus if some users prefer that format they can use it.
However I did not want to modify this style with breaking changes by introducing the options for sections there.
I also did not add the help output for this to the Readme in order not to confuse new users who can start with the new subcommands. But if you just run gci -h you still see all the old options:

Usage:
  gci [-diff | -write] [-local localPackageURLs] path... [flags]
  gci [command]

Available Commands:
  completion  Generate the autocompletion script for the specified shell        
  diff        Prints a git style diff to STDOUT
  help        Help about any command
  print       Outputs the formatted file to STDOUT
  write       Formats the specified files in-place

Flags:
  -d, --diff            display diffs instead of rewriting files
  -h, --help            help for gci
  -l, --local strings   put imports beginning with this string after 3rd-party p
ackages, separate imports by comma
  -v, --version         version for gci
  -w, --write           write result to (source) file instead of stdout

I think this is the best compromise for adding this new functionality. This way the cli that previous users are accustomed to is still present. If you want to use the new functionality you can just use the new commands.
Another benefit of the new subcommands is that they have autocomplete support. Just writing gci w<TAB> which gets autocompleted is even faster than writing gci -w.

[daixiang0]

Great, let's make it land.

[daixiang0]

Also please sign all your commits, you can refer to this doc.

[daixiang0]

@ngehrsitz thanks for your great contribution!