Make it configurable whether +patch is assumed fallback.

[JKrag]

When I made this, I assumed that we always want to default to +patch and only have users specify major and minor.

I have since considered that this can be unsafe as we would potentially release bigger changes as patch bumps.

I would like to make it an optional feature. Ideally support the following modes:
a) Require a bump in every commit - else fail.
b) Default to patch bump (previous behaviour)
c) Default to major bump (the paranoid approach - if the developer hasn't explicitly stated otherwise, we assume that he broke something)
d) Default to minor bump - mostly for completeness, but potentially quite useful for 0.x.x initial development.

[JKrag]

For ver. 2.0 we are targeting the following plan:

Default behaviour is a) fail if missing.

In #15 we have described optional flags for specifying a default bump level to use if none is found in a commit.