Specify extras_require in setup.py

[chrahunt]

Currently hachoir-urwid and maybe other utilities have unstated external dependencies. For hachoir-urwid this is a problem because doing a plain pip install urwid will install version 2.x which seems to be incompatible (see #34). If we pass something like

'extras_require': {
    'urwid': [
        'urwid==1.3.1'
    ]
}

to the setup method in setup.py then users could pip install hachoir hachoir[urwid] and be assured they get a tried and tested version.

[vstinner]

Sure, go ahead with a PR!