Allow selectors on next line

[dmitry-timofeev]

Allows targeting expressions to be on the next line:

<!--codeinclude-->
<!-- On the same line -->
[Title](long/long/Path) targeting_expr1

<!-- On the next line -->
[Title2](long/long/Path)
targeting_expr2
<!--/codeinclude-->

I kept the usage of a regular expression to find the include definitions, but had to change the implementation to use re.findall and a multiline expression instead of re.match over a single line.

The processing is now separated into three stages:

1. Code-include block extraction.
2. Computing substitutes (patches) for each block.
3. Application of patches to the original markdown.

Resolves #9

[dmitry-timofeev]

Just in case it helps with the review, I keep a develop branch with all the open PRs merged into it (and merge conflicts resolved): https://github.com/exonum/mkdocs-codeinclude-plugin/tree/develop

[rnorth]

Trying to unpick these and forming a larger PR, #22, where I'll resolve the conflicts that have arisen