Allow '*' in ycm_filetype_specific_completion_to_disable.

I love YCM's auto popup with keyword completion when power typing but
would rather not have semantic completion activate unless I really want
it to (most semantic completers are somewhat slow and sometimes end up
breaking my flow). When in API exploration mode, I don't mind having
to press C-Space to force completion and wait a bit more.

Unless I'm mistaken, the current code does not allow wildcards in the
specification of filetypes on which to disable automatic semantic
completion. This change allows the use of '*' to disable automatic
semantic completion in all files.
Document '*' in ycm_filetype_specific_completion_to_disable.