Allow passing a preamble file to the CLI

[borgstrom]

This PR adds a new CLI option, -p or --preamble-file, that is used to provide a preamble to the pex builder object.

We use pex via its API in the internal build tooling at NerdWallet (👋, we're neighbors!). We've been leveraging it for multi-platform support, but now that #394 / 1.2.9 has landed the only thing missing from the CLI that we leverage is the preamble in the builder, which was pretty easy to hook up. Once this PR has landed we can remove all of the API integration and just rely on the pex CLI!

The CLI option and the PEXBuilder preamble functionality are covered by tests.

Local functional testing:

(pex-foo) 
evanborgstrom@evanborgstrom /tmp/preamble — u:34 j:1 (21:49:54 08.16)
#536 ❯❯❯ echo 'print "foo!"' > preamble
(pex-foo) 
evanborgstrom@evanborgstrom /tmp/preamble — u:34 j:1 (21:50:11 08.16)
#537 ❯❯❯ pex -p preamble 
foo!
Python 2.7.13 (default, Jun  7 2017, 11:02:53) 
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> 
(pex-foo) 
evanborgstrom@evanborgstrom /tmp/preamble — u:34 j:1 (21:50:18 08.16)
#538 ❯❯❯ pex 
Python 2.7.13 (default, Jun  7 2017, 11:02:53) 
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> 

[jsirois]

Yeah, how about via execution of a built pex with pre-amble? If the preamble echoed, say and exited with a known exit code you could test bout the stdout and the exit code are as expected.

[borgstrom]

@jsirois -- Done. 👍

[kwlzn]

this LGTM - thanks for the PR!

will plan on merging this tomorrow afternoon to allow for any straggler follow-up comments.