Only report filename once as opposed to once per error. Fixes GH-117

The "Linting filename ... ERROR" message is now printed once per file, rather than once per error/warning.  Fixes GH-117.

In other words, instead of 

  Linting dir/file1.js ...ERROR
  [L1:C2] E001: Foo
  invalidCodeHere
  Linting dir/file1.js ...ERROR
  [L2:C1] E002: Bar
  invalidCodeHere

You would now see

  Linting dir/file1.js ...ERROR
  [L1:C2] E001: Foo
  invalidCodeHere
  [L2:C1] E002: Bar
  invalidCodeHere