Bump version command

[AaronAsAChimp]

Add a command to bump the version number of a project.

• Look for a VERSION.txt in the root
• Parse it according to the semantic versioning rules
• increment a part of the number

$ cat VERSION.txt
0.3.4
$ release-tool bump minor version
$ cat VERSION.txt
0.4.0
$ release-tool bump major # the version parameter is just for syntactical sugar
$ cat VERSION.txt
1.0.0