Add ability to ignore lines

[tntim96]

How (and if) this is implemented is currently wide open. The JSCoverage approach has been partially implemented, however this only works in the JSCoverage report, so am not keen to add something limited in this way except for those migrating from JSCoverage.

Things to consider:

• How should merging be handled
• Should statistics be affected (i.e. add a fake increment or teat ignored code as comments)
• How could this work in other reports (e.g. LCOV)