Add a title attribute to lexers

The title is meant to be a human-readable name for the language. i.e. "C++"
instead of `cpp`. When the gem is initialized the titles are all set to the ivar
@title. If no title is set, it will use the demodulized class name:
`Rouge::Lexer:XML` will become "XML".

My text editor also stripped out some whitespace. I can put that back if it's important, but it looked like mistakes to me.

:christmas_tree: 
either way I think a reasonable default would be `tag.capitalize`.