Add some built in validations

[DannyBen]

For #125

• integer
• not_empty (assuming Allow empty string as flag argument #122 will be merged)
• file_exist
• git_branch_exist ? (not sure this is important)

We should only add common ones and leave the niche ones for users to implement on their own.

[wolfgang42]

I intended git_branch_exists to be an example of an uncommon validation that a user might want to implement themself, to contrast it against e.g. not_empty which seems like it should be a builtin.

[DannyBen]

Excellent. If you have other validation types you think should be included, feel free to list them.

I am assuming that we first need #122 before proceeding here, right?

As a general rule, I usually try to not let things linger too much in master branch (before they make it into a release) - so I hope we can tie loose ends for the validate feature one way or another, soon.

[wolfgang42]

I can't think of anything immediately, I'll start a discussion if anything else occurs to me. Agreed it'd be nice to get the loose ends cleaned up, hopefully ones #122 lands I can also clean up that set -u PR that's been languishing.

[DannyBen]

Closing. This is implemented in #136.