Add argument name to Options class and make it accessible from the argument dict

[con-f-use]

Consider the option strings:

-o --option=<int>         An Option
-a --another=<float>   Another Option

For post-processing of command line arguments it is sometimes useful to have the argument type available (meaning <int> or <float> in the example above).

E.g. for things like:

    args = docopt(a,[])
    for k, v in args.iteritems():
        atype = [t.atype for t in args.options if t.long == k]
        atype = atype[0] if atype else None
        if atype == '<int>':
            args[k] = int(v)
        elif atype == '<float>':
            args[k] = float(v)     

My pull requests adds the atype property to the Option class which stores the argument type (or whatever <int> and <float> might be called in above example strings). It then makes the results from following line in docopt(...) available:

options = parse_defaults(doc)

It can be accessed from the argument dictionary like so:

from docopt import docopt
args = docopt(a,[])
args.options

Everything else remains unchanged.

[con-f-use]

The travis "All checks have failed" doesn't seem to come from my changes:

0.19s$ pip install tox --use-mirrors
Usage:   
  pip install [options] <requirement specifier> [package-index-options] ...
  ...
no such option: --use-mirrors

[con-f-use]

@mboersma yes? no? maybe?