Allow specifying multiple paths as positional arguments

[guykisel]

This is related to #56. It would be easier to wrap prospector in other tools (such as a git commit hook) if it could accept more than one path at a time.

I'm interested in implementing this myself and submitting it as a pull request, but I wouldn't mind if someone else with more free time does it instead.

[guykisel]

My investigation so far makes me think that the command line config settings already support multiple positional input paths, since prospector is passing nargs: * into setoptconf, which passes it into argparse, which interprets it according to https://docs.python.org/2.7/library/argparse.html#nargs

positional = (
        ('checkpath', {
            'help': 'The path to a Python project to inspect. Defaults to PWD'
                    '  if not specified.',
            'metavar': 'PATH',
            'nargs': '*',
        })

I think the key is that currently prospector only uses the first path in the list of paths:

    # Make it so
    prospector = Prospector(config, paths[0])
    prospector.execute()
    prospector.print_messages()

and Prospector is built around taking in a single path at initialization.

The solution seems to be either:

1. Rewrite Prospector to allow specifying a list of paths
   or
2. Iterate over the input paths, creating a new Prospector for each one and aggregating the resulting messages, then print a single summary for all of those messages.

[guykisel]

Opened #78 for this issue.

[carlio]

I merged in #78 and you can now run prospector with

prospector file1.py path/to/file2.py ...

It will output all messages together followed by a summary. It's sort of equivalent to saying prospector . then explicitly ignoring any files you don't want checked.