Add num_perturbations param to all langcheck.augment functions

[kennysong]

This PR adds a num_perturbations param that allows you to generate multiple perturbations for each string.

For example,

>>> langcheck.augment.synonym('the red fox jumps over the fence', num_perturbations=5)
['the red dodger jumps over the fence', 'the ruby red fox jumps over the fence', 'the red fox jumps over the fencing', 'the red fox jumps concluded the fence', 'the red fox jumps over the fencing']

Also tweaks the docstrings to be more consistent with each other.

[kennysong]

Change to keyword only params

(accidental comment)

[Alnusjaponica]

LGTM. Thanks for correcting some expression.