Find rebar.config, new options (outdir, xref, load, copy)

[hcs42]

The main new features in this PR are the following:

• Find rebar.config automatically.
• With --outdir DIR, we can generate a beam file.
• With --xref, we can execute xref.
• With --load, we can load the module into the given node.
• With --copy, we can overwrite the old beam files with the new one.
• New command: ErlangToggleShowErrors.
• ":make" and flymake can be configured differently with different global variables (g:erlang_make_options and g:erlang_flymake_options), because one might use them for different purposes. (E.g. flymake is expected to be quicker, so one might want to use the "--load" and "--copy" features only with ":make".)

All these new feature are turned off by default, except one: when the user uses ":make", the beam file will be generated next to the Erlang source file. I think this is what Vim users generally expect from ":make", not just a syntax check. (Note that flymake will still not generate a beam file by default.)

Other improvements:

• Add type specs and docs to each function.
• Add some pointers to the README.

[hcs42]

I'm merging this PR because I'm pushing a new branch on the top of this one.

[aerosol]

:+1: