Add C lexer

[gandarez]

This PR adds C lexer as well its analyzer as implemented in Pygments.

https://github.com/pygments/pygments/blob/master/pygments/lexers/c_cpp.py#L313

[alecthomas]

I wonder if it might not be a good idea to extend the XML format to include something like:

<analyzer regex="..." score="0.2" />

[gandarez]

 regex="..." score="0.2"

What do you mean by score? The result of AnalyzeText may vary according to some checks.

[alecthomas]

Yes I'm aware of what it can do, I'm saying that for the common case where you have a couple of regexes and corresponding scores, that could easily be defined declaratively in the XML without having to write a Go wrapper.

[gandarez]

Some analyzer don't use regular expressions, like for BBC Basic:

if strings.HasPrefix(text, "10REM >") || strings.HasPrefix(text, "REM >") {
	return 0.9
}

return 0