Allow source argument for update

[lhupfeldt]

Actual Situation

The template repository may be moved or renamed, preventing update.
Currently you have to hack the answers file (which is forbidden), and commit it before updating.
Very inconvenient, also during template development when you are working with a locally checked out template and end up with a file system path in a subproject.

Desired Situation

No hacking needed to change template repo location.

Proposed solution

There should be an optional --source argument to point to the new location during update or recopy.

[lhupfeldt]

Related to #335

[yajo]

Fir recopy, just copy again. Recopy is just a shortcut for that.

In the case of updating, it would make sense.

[lhupfeldt]

Fir recopy, just copy again. Recopy is just a shortcut for that.

In the case of updating, it would make sense.

I would say just very important shortcut. In scenarios where you actually recopy is is not only annoying, but also error prone to have to specify all arguments again. And why would you implement the option for update, but not for recopy at the same time? I would assume the added complexity would be very low, zero or even negative because of more code reuse.
I would also say that the interface would be less confusing to users by having this option on both commands.

[yajo]

A hypothetical command like this:

copier recopy --src new_src dst

Would be exactly the same as this:

copier copy new_src dst

So, I don't think there's much value on supporting a duplicate feature.