set-property should add the property if it does not yet exist

[mandritschke]

Fixes #1268

• instead of just updating existing properties allows to insert completely new properties new properties overriding properties from parent POM
• works for properties on the project level as well as on the profile level (that is if a profile id is given)
• though this implementation won't create new profiles at the moment, this could be a possible enhancement, let me know if this sounds useful
• formatting was kind of a challenge

Usage

mvn org.codehaus.mojo:versions-maven-plugin:2.21.1-SNAPSHOT:set-property \
  -Dproperty=mockitoVersion \
  -DnewVersion=4.11.1 \
  -Dinsert

Notice that set-properties will be executed recursively on child modules if there are any.
If you don't want the property to be inserted into all of these child modules use the -N option.