Several CLI improvements

[joshkel]

This PR is a bit of a grab bag: I don't know if you'll want all of these changes as is or not.

Part of this is similar to #11 (which I hadn't seen before I put this together); one difference is that this accepts passing a single file containing JSON sample object or multiple files containing JSON arrays of sample objects, while #11 assumes that a single parameter means a JSON array of sample objects and multiple parameters mean multiple files containing JSON sample objects.

Specific changes:

Shorten the help text; previously, yargs wasn't leaving a space between a parameter's help text and its type (classes[string]).

Fix a misspelling in help text.

Show a warning if neither -i nor -p is specified, to make the command a bit less confusing for those who don't read the help.

Allow providing multiple filenames as input. (See #10.)

Allow input filenames to consist of a single JSON sample object as well as an array of JSON sample objects.

[coveralls]

Coverage remained the same at 94.3% when pulling bb5d5f3b5e79f3980535c9390c46aa19f1831eab on joshkel:cli-improvements into 9d57d7b03f9dded0556dfb067896b09c6b860891 on jvilk:master.