Fix "makeprg" in erlang.vim

Previously, ":make" didn't work, because the name of the current file was not
passed to the erlang_check script.
Find rebar.config automatically
Improve rebar option processing

- lib_dirs are used only for their include dirs now, and sub_dirs are used for
  their code path.
- Remove the "warnings_as_errors" item, because we don't want that during
  compilation with Vim. (See more info in the commit body.)
Use "{capture, none}" in re:run
Check the return value of "escript -s" instead of analyzing its output
Add type specs and edoc
check_file now returns ok instead of {ok, term()}

This commit is just a refactoring. I made {ok, term()} into 'ok' because nobody
used the term returned.
Simplify main function
Add --verbose option and help text
Add --outdir option

The --nooutdir option may be useful when we already generated a command that
includes "--outdir", and we want to override it by only appending new options.
Add ErlangToggleShowErrors command
Remove "silent" from Erlang(Dis|En)ableShowErrors
Indent ftplugin/erlang.vim as the other files
Generate beam for :make (but not for flymake)

The above options are the default, both can be customized as described in the
help text.
Add cpo header/footer
Add TOC to doc and move the credits to the bottom
Add "Installation" to README
Add doc link to README
Add --xref option to print calls to undefined functions
Add --load option to load the module into a node