Improve performance by making LintTask incremental

[mateuszkwiecinski]

Looking at the source code, running the lintKotlin will result in all the sources being checked on each task re-run.

To improve the performance, LintTask could run against changed files only (documentation)

If this gets implemented, a similar thing could be adopted for formatKotlin task (with slightly bigger effort due to overlapping inputs and outputs), but could help implementing: #155 (or not? not sure what's the desired direction there)

[jeremymailen]

Yes, this one is on my wishlist. We're incremental at the sourceSet level, but if a sourceSet is large we might get a boost from only processing changed files in the sourceSet.