ExtractorMojoHelper - replaces all {a|b|c} expressions in a value.
ExtractorMojoHelper - {a|b|c} expressions require 2 variables at least, no replacement is made otherwise.
ExtractorMojoHelper - helper.updateValue() is used when handler values are read.
ExtractorMojoHelper - helper is initialized last.
Merge pull request #12 from evgeny-goldin/maven-plugin

{a|b|c} expression handling - last variable is a default value