Add register keyword for adding customized parsers and formatters

[namusyaka]

ref 93e6998 #1329
I'd like to use grape-msgpack without tricky hacking. Currently, our implementation does not support inheritable interface, so I fixed a few points.

Thoughts?

[dblock]

I see how a require fixes this, but we now load formatters we don't use. Why doesn't autoload work with grape-msgpack?

[namusyaka]

Nope, I removed autoloads to declare FORMATTERS and PARSERS constants.

[dblock]

Sorry, I am a little slow, and clearly I am missing some Ruby-fu understanding :)

Why doesn't it work with grape-msgpack?

[namusyaka]

At first, grape-msgpack didn't consider the commit.
However, something like this line has broke inheritance interface.
Our provided options cannot be inherited, so if we want to implement customized grape formatter, we must require the tricky code.

I think we should be able to provide more developer-friendly interface.
If this PR is merged, maybe grape-msgpack's can be resolved by a little bit changes.

[dblock]

What if we just added a global interface to register formatters, instead of inheriting anything? Like so:

Grape.configure do |config|
   config.formatters.add
end

[dblock]

Or

class MyFormatter << Grape::Formatter
end

Grape::Formatter.register MyFormatter, ...

[namusyaka]

Yeah, it's so cool and developer-friendly. :)
Btw, should we contain the configuration feature in 0.16.0?
Also, can this PR be used as a 0.15-backported patch?

2016年3月17日木曜日、Daniel Doubrovkine (dB.) @dblockdotorg<
notifications@github.com>さんは書きました:

What if we just added a global interface to register formatters, instead
of inheriting anything? Like so:

Grape.configure do |config|
config.formatters.addend

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#1330 (comment)

[namusyaka]

Hmm, right. It's reasonable.

2016年3月17日木曜日、Daniel Doubrovkine (dB.) @dblockdotorg<
notifications@github.com
javascript:_e(%7B%7D,'cvml','notifications@github.com');>さんは書きました:

Or

class MyFormatter << Grape::Formatterend
Grape::Formatter.register MyFormatter, ...

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#1330 (comment)

[dblock]

I don't think we should backport anything and just move forward. If you want to give one of the above a short, go for it! I'd rather not remove autoload. I prefer the second version btw, not the global configuration one because that always bites us.

[Vsandhya11]

I am planning to write micro service using ruby-grape, So I am thinking 0.15.0 is stable version to use. Plz lemme know your suggestions.

[namusyaka]

I've tried to implement register keyword on those by using the second feature as base approach.
@dblock Could you confirm the changes?

[dblock]

It's not clear what's being fixed. The built in parsers and formatters weren't broken really.

[dblock]

I'll merge this as is, feel free to make further changes later. Also I think we don't need to document this publicly for now.

[dblock]

Add a similar spec for parsers.