filter site content without hijacking main resources

[hboutemy]

fixes #759

this is the filtering strategy that uses explicit maven-resources-plugin execution (bound to site lifecycle) instead of hijacking main resources

[coveralls]

Coverage remained the same at 89.404% when pulling 517caba on hboutemy:site-resources into f7e2020 on mybatis:master.

[michael-o]

This is a good idea when you don't need all features of Velocity.

[hazendaz]

@hboutemy Thanks!