Add ability to specify a value of a part of version only

[alexandrnikitin]

Similar to --new_version=X.X.X, it can be --new_version-part=X parameter that tells the value to use as the part of the version. Other than specified part values will be taken from the current version.

# Say current_version=0.1.0
bump2version --new-version-part 3 patch
# will set 0.1.3 version value, where major and minor are taken from the current version

Right now I need to write bash script to handle that.

[alexandrnikitin]

Closing as duplicate of #50