Error Checks block increases incremental build time

[andyzg]

During ts-loader's after-plugin callback, the error checks take a ton of time per incremental build. In the code base I'm currently working with, error checks suck up over 6 seconds of incremental build time.

All typescript files by default are currently checked for errors. I'm proposing that we add a regex to exclude files solely for error checks for incremental build time optimization.

My current build time went from 8s to 2s on average through a prototype of this change.

[andyzg]

Duplicate of #78