Check vim-erlang-tags options on each call

If pathogen is used to handle plugins in conjuntion to a autocmd for
configuring the plugins on load (to be able to detect if a plugin exists or not
as pathogen loads them after vimrc has been read) the script_opts would not
have been set and calling ErlangTags would ignore it.
Remove script scope from script_opts variable
Merge pull request #5 from sedrik/check-options-everytime

Check vim-erlang-tags options on each call
Add g:erlang_tags_outfile option to specify tags output file