Adding thrift-linter to pants.

Background and some discussion: https://groups.google.com/d/topic/pants-devel/xaYZ4ezUqok/discussion

thrift-linter calls scrooge-linter to detect problematic usage patterns in thrift files and fails if linter errors are found.

The linter has a strict option. If false, it prints the linter messages but does not fail the build.

The strict option can be set from command line (--[no-]thrift-linter-strict flag), in a BUILD file (java_thrift_library(... thrift_linter_strict=False|True)), or in pants.ini, in [thrift-linter section].

One requirement is to be able to call the thrift-linter goal separately. So it has to be a top level goal rather than a part of gen.

Testing Done:
build-support/bin/ci.sh
https://travis-ci.org/nshkrob/pants/builds/35587177

Reviewed at https://rbcommons.com/s/twitter/r/1044/