Make snippet plugin configurable

Code for interfacing with snippet plugins can be placed in
autoload/clang_doxygen_snippets.
As a default none.vim is provided that inserts plain doxygen comments without
any placeholders.
Add support for UltiSnips
Add support for XPTemplate
Update documentation

Add section on configuration options.
Update documentation and README
Skip over NamespaceRef
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
Update README