Re-evaluate fast functionality

[nycdotnet]

The current fast and watch functionality may possibly be improved by considering integration with --isolatedModules and the native watch functionality. Perhaps not. Any thoughts are welcome.

//cc: @basarat

Known bug: .tscache files are still written to disk even in the presence of a testExecute value.
Issue: Very noisy (#191)
Watch functionality does not provide errors if given invalid values (such as a glob).

[jbrantly]

As I understand it grunt-ts actually executes tsc directly. I could be mistaken but I'm not sure that --isolatedModules actually provides any speed increase unless you're using the transpileModule TS API. I'm purely basing this off the comments here and here. Namely:

the idea is you would use this with in a normal compilation, with multiple files, to ensure you can do the compile them in isolation later, hence the "s".

[basarat]

Yeah, isolatedModules is mostly for error reporting about someone using things that not safe for transpilation. For fast we only give TypeScript one file ... so will not have any impact