Removed code duplication and added features to ValueModifier

This commit uses templates to reduce code duplication in
ValueModifier. enumerate() now supports being given a base string that
it will append its numbers to. There is also a new method --
enumerateAlpha() -- which will take a string and use its lexiographic
incrementation instead of a number. enumerateAlpha also supports a
base string.

This patch was created by andromeda-galaxy within GCI 2012,
thanks a lot!