Parsing static resources spec.

• Line starting with word set is start of ResourceSet definition.
• Line starting css, js or jsx defines Resource.
• Line starting with ignore defines that some file should be ignored by checker.
• ALL other lines are ignored

Sample spec

ignore main.css

set a
  js  a.js 
  css a.css
  css common.css

set b
  js  b.js    
  css b.css
  css common.css