add dialect filter

[MiniXC]

As pointed out in #4, some languages are made up of a group of specific dialects in phoible.
We need to figure out how to communicate this best and have to add an option to filter by dialect to phones.

I think it might be best to go with something like this.

from phones import PhoneCollection
pc = PhoneCollection()
pc.langs("ast")
>>> ValueError: Need to select a dialect for "ast". Dialects can be listed using the list_dialects flag
pc.langs("ast", list_dialects=True)
>>> ["Asturian (Western)", "Asturian (North-Eastern)"
pc.langs("ast", "Asturian (Western)")

It would be nice to allow something like pc.langs("ast", "western"), which could be achieved by just checking if the dialect string only occurs in one of the dialect options.

[MiniXC]

implemented in 0.0.3