Use findfile() and added documenation

[PeterRincker]

I have added the following:

• findfile() in cscope_auto#locate_database()
• support for g:cscope_auto_database_name or b:cscope_auto_database_name
• Added documentation

[ktchen14]

@PeterRincker Whoa thanks for adding documentation for this! The only thing I'm not sure about is whether or not I want to go with findfile() at the moment. The reason is that I'm considering (optionally) making cscope_auto_database_name a list such that either:

let g:cscope_auto_database_name = '.cscope'

Or:

let g:cscope_auto_database_name = ['.cscope', 'mycscope.out']

Would be valid. In the latter case cscope_auto#locate_database() would perform an upward recursive search and would use the first file found called either .cscope or mycscope.out with preference given to .cscope if both are present in the same directory. As far as I can tell this isn't something I can do with findfile(); for example if we have:

/a/b/.cscope
/a/b/c/mycscope.out

And the base directory is /a/b/c then I'd expect to use /a/b/c/mycscope.out since it occurs earlier in the search order. Using findfile() and iterating over the list provided we'd choose /a/b/.cscope instead without some additional logic to compare the lengths of the paths returned by each name.

My thinking is that this may satisfy most use cases requiring a b:cscope_auto_database_name in that you'd just define all the names that you're interested in using once.

[ktchen14]

Drop the findfile() commit at 692afd7