A few 'mix escriptize' fixes:
A few 'mix escriptize' fixes:

* Check timestamps of .beam files before generating escript.
* Don't generate an escript if no .beam files exist.
* Add --force option to disregard timestamps.
* List command line options in docs.
* Correctly specify --no-compile as a switch to OptionParser.

Fixes #760.

beedub/elixir,beedub/elixir,pedrosnk/elixir,gfvcastro/elixir,joshprice/elixir,ggcampinho/elixir,gfvcastro/elixir,lexmag/elixir,pedrosnk/elixir,kimshrier/elixir,antipax/elixir,kelvinst/elixir,kelvinst/elixir,antipax/elixir,ggcampinho/elixir,kimshrier/elixir,elixir-lang/elixir,michalmuskala/elixir,lexmag/elixir