Add 'fromfile_prefix_chars'

[nodeca]

Add 'fromfile_prefix_chars'

[hpaulj]

https://github.com/hpaulj/argparse/tree/from_file

is a branch with a preliminary implementation of the fromfilePrefixChars feature. Most of the change is in the
ArgumentParser.prototype._readArgsFromFiles function. It works but needs to be cleaned up because it is
a translation of a coffeescript version.
test/test_fromfile.js is a test derived from the python test file. It is not in mocha format yet. It sets up a /tmp dir
and writes 3 small files to that. It should clean up after itself.

For file access I am using the Sync versions of the node fs file functions.