Add drop pre task

[mpanarin]

Closes #23

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 78.64%. Comparing base (f3e8e34) to head (b95de72).

Files	Patch %	Lines
lib/Tasks/mix_bump.ex	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   78.53%   78.64%   +0.10%     
==========================================
  Files          17       17              
  Lines         205      206       +1     
==========================================
+ Hits          161      162       +1     
  Misses         44       44              

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[mpanarin]

@kaharlichenko please try this version if it properly fits your purpose.
You should be able to bump from 1.2.0-alpha to 1.2.0 via mix bump next.
Feel free to ping me here if anything is wrong.

Beware of some breaking changes from the latest release https://github.com/mpanarin/versioce/blob/master/docs/migrations.md#migrating-from--300

[kaharlichenko]

Thank you for the update. I played with this version and indeed I can change from 1.2.0-alpha to 1.2.0 via mix bump next.

On the other hand the next bumping mode is somewhat confusing.

Based on its name I would assume that it always advances the version. This doesn't seem to be the case, at least with respect to Semantic Versioning 2.0.0.

Here are the test cases that I ran:

Initial Version	Command Run	Resulting Version	Advances (per semver)?
0.1.2	mix bump next --pre alpha	0.1.2-alpha	👎
0.1.2-alpha	mix bump next --pre beta	0.1.2-beta	👍
0.1.2-beta	mix bump next --pre alpha	0.1.2-alpha	👎
0.1.2-alpha	mix bump next --pre alpha	0.1.2-alpha	👎
0.1.2-alpha	mix bump next	0.1.2	👍

What I'm trying to say is that on one hand it handles my particular use case. On the other hand the naming next might be confusing for users as it doesn't guarantee that the version is going to be advanced, rather changed.