tsrc sync -r support

[vonpupp]

Hello,

Following up on #359, I took the liberty of submit this PR to add -r support to the sync command.

I had to take decisions because of the -r arg collision, so I decided to change the regex inclusive arg to -i and the exclusive arg to -e, which made sense to me.

I ran lint and tests. Everything seems to be okay. I also fixed some typos in the doc regarding multiple remotes.

I hope this makes sense and gets accepted. Best regards.

[vonpupp]

I forgot to mention. I didn't update the change log file, since it is not included in contrib/dev.md guide and I am not sure how the releases are handled nor if the PR is going to be accepted.

[gdubicki]

I resolved the conflicts and did a small refactor that I felt was necessary: after the cli arg rename from -i to -e the iregex name became inconsistent and misleading. Please see 2c749a1 to check what I did with it.

I think that to merge this PR we just need to resolve that single review comment. Can you do that please, @vonpupp?

It would be great if you could also add a test for the new cli arg for tsrc sync, but I think that because we kept you waiting for so long here, to follow the essence of the optimistic merging, we can accept not adding this test. Just let us know what you choose!