Add configuration options for comment generation

It is now possible to configure the tags that are used and also the kind
of comments, i.e. C block comments (/** .. */) or C++ comments (/// ..).
Add comment generation for class declarations
Rename block_comment to use_block

The option g:clang_doxygen_block_comment is now called
g:clang_doxygen_use_block.
Add documentation for style options