Implement Projbook's Snippet include feature

[FransBouma]

Projbook supports inclusion of snippets from external files. This is particularly helpful if you e.g. want to include a method or class from a C# file into documentation: by using the inclusion of a snippet, the original text is included as a fenced codeblock and it's not a burden to keep things in-sync.

The proposed syntax is:

@snippet language [filename] pattern

As Projbook is MIT licensed, we can include their extractors for C#, XML and plain text, and use that. The C# extractor is particularly helpful as it's based on Roslyn and supports some neat syntax. See: http://defrancea.github.io/Projbook/projbook.html#Pageextractormd-c--pattern

In the syntax above:

• language is either cs, xml or txt. If an unknown string is specified, txt is assumed. The language is used for which extractor is used, and also for the language for the resulting fenced code block.
• filename is a filename to the file to read the snippet from, which is relatively specified to the markdown document the statement is specified in.
• pattern is the pattern to pass to the extractor and this follows the Projbook syntaxis.

Tasks:

• [X] Implement code
• [x] Add @snippet statement to docs
• [x] Add ProjBook theme

[FransBouma]

Implemented