Add command triggering tag generation. Details:

1. The plugin will now load only once.

2. ErlangVimtags was chosen instead of VimtagsErlang for the
command name. It's a matter of personal preference, but I'd
risk saying it might be easier to associate 'E' than 'V' with
Erlang-specific tasks

3. The plugin will report errors returned by the escript, if any.