Allow global configuration of tslint.json

[cyrilgandon]

It's nice to have a configuration by projects of tslint rules, but it can be useful to have only one set of rules for every projects that are gonna be analyze by one instance of SonarQube.

[Pablissimo]

It's not a bad shout. At the same time we'd have to make the custom rules directory globally-configurable too, but both are trivial changes. Will see about it tomorrow.