Change --tag option from bool to template in project:bump command

[b0g3r]

Closes #197

I have questions about three things:

1. CONTRIBUTE.md doesn't say anything about running modified dephell for manual testing, only running checking and testing. Now I'm using poetry run dephell command, but I'm not sure about it.
2. I don't see any tests for argument schema validation. Why?
3. I have no idea about git tag creation testing :( I tried to search, but found only tests for git package resolving.

[orsinium]

Beautiful. Thank you!

I think, we can use {version} instead of {new_version}. Also, if placeholder is not specified, we can interpret it as a prefix. For example, --tag="v."

[b0g3r]

Good proposal, I implemented the prefix feature! But I concerned about testing git tag feature, how can I test it?

[orsinium]

Oh, that's a long story. We have to make temporary directory, git init, run command and check commits. However, we have no this kind of tests yet, it's in future plans. I've commented it in #167, and then we lost contributor. So, I can accept PR without tests and add it in the scope of future works about this kind of tests.

[b0g3r]

Added tests!

[orsinium]

Wonderful! It's a big step to the perfect tests and important contribution.

I've added a few comments on how to make it a little bit better. However, important only 2 of them: about the error message and about documentation. It's the things that users see. Everything isn't really important, but it would be cool if you've done it BTW.

[orsinium]

Perfect, thank you :) Tonight Travis CI has troubles with Linux build, so, I'll merge it tomorrow, if tests works

[orsinium]

Thank you, nice job.

If you'll have time and willing to help more, we have a bug #127 related to the usage of .git in some tests. If you propagate the way of testing from this PR on other git-based commands, it will work. Feel free to contact me or open an issue if you have some questions or bugs.