Add support for libgit2 feature detection

[carlosmn]

This lets us ask the library whether it supports threading, https and
ssh.

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I wasn't sure whether to leave it with the int as-is or convert the result to a list of strings, so you can do

if 'ssh' in pygit2.features:

instead of

if pygit2.features & pygit2.GIT_FEATURE_SSH:

The former seems more in line with what a pure-python library would do, but we do expose the underlying flags in most places.