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

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.