Add keyword-aliasing for underscores

[hynek]

This fixes #6 by stripping underscores from attributes for the initializer keyword-arguments.

It does it only for the new-style Attribute classes because I have the feeling that adding it retroactively to string-based ones would break backward-compatibility big time (I’ll happily listen to contrary opinions :))

What this does is basically the following:

@attributes([Attribute("_private")])
class C(object):
    pass

c = C(private=42)
assert c._private == 42

I would appreciate both feedback whether this is what you wanted and for a better name for keep_underscores because the name is gross. :|

[coveralls]

Coverage remained the same when pulling 7f777eb on private-attrs into 6ff5302 on master.

[coveralls]

Coverage remained the same when pulling 32f3158 on private-attrs into 6ff5302 on master.

[coveralls]

Coverage remained the same when pulling 9fbbb1e on private-attrs into 6ff5302 on master.

[coveralls]

Coverage remained the same when pulling c072caf on private-attrs into 613f71c on master.

[coveralls]

Coverage remained the same when pulling 6e307dc on private-attrs into 465d0e1 on master.

[hynek]

Not waiting any further for feedback. If this is not what you wanted, re-open and comment on #6.