Add ability to optionally ignore a directory

vim-erlang-tags.erl now takes a `--ignore Name` option (or `-i Name`), which it will ignore. `Name` is expanded using `filelib:wildcard/1`, so it can be a glob-match. This can be specified multiple times for multiple ignores. It can be a filename or dirname.

vim-erlang-tags.vim: specify `g:erlang_tags_ignore` in vimrc as follows:

     let g:erlang_tags_ignore = "_rel"

Known issues: can only specify one ignore pattern in vimrc