TSLint: add ability to exclude files and folders

[datencia]

Short description of what this resolves:

After upgrade to Ionic 3, tslint shows errors even for external typescript libraries inside the node_modules folder.

Changes proposed in this pull request:

• Change envVar option value to upper case (ionic_tslint -> IONIC_TSLINT) (as the rest of functionalities)
• Change packageConfig option value to lower case (IONIC_TSLINT -> ionic_tslint) according to the docs Overriding Config Files
• Add ability to override config file for linter (actually, the package config setting is only used to specify the tslint.json to be used. Now allows you to:
  • Exclude files and folders from the linter by setting glob patterns
  • Specify the tslint.json file that is used.

Fixes: #1203

[yannbf]

Looking forward to this getting merged! I'm having issues and this blocks the usage of a few pre-commit hooks in our project.

Could @danbucholtz take a look? It's been two months :s

[MattiLehtinen]

Would appreciate this!

[miquelferrerllompart]

Would appreciate this too!