Avoid throwing in ApacheThriftGen.__init__

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/43046341

Bugs closed: 842

Reviewed at https://rbcommons.com/s/twitter/r/1428/
Support option reads via indexing.

This will support tasks with table-driven logic.

Testing Done:
$ pants.dev goal test tests/python/pants_test/option

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/43132907

Bugs closed: 846

Reviewed at https://rbcommons.com/s/twitter/r/1435/
Properly detect existence of a symlink.

os.path.exists() returns False for broken symlinks.

Reviewed at https://rbcommons.com/s/twitter/r/1437/
Reintroduce rcfiles.

Now they are simply alternate config files, in pants.ini format.

This also re-implements the "overrides" functionality.

This change also modifies how config caching is done. Previously
configs were keyed by the file they were read from. But in practice
code always calls Config.from_cache() with no argument, which
defaults to the regular pants.ini. This leads to dangerous confusion
when config can be read from other files. So now there's just a single
global cached config, which must be set explicitly.