Fix tsconfig.json exclude support to support files

[nycdotnet]

Officially, the tsconfig.json exclude property can support file names. Current implementation in grunt-ts only supports directories. (https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#exclude-property-support-in-tsconfigjson)

[ghost]

👍 This is preventing me from excluding the typings/main.d.ts file generated by typings, which results in a long list of duplicate definition errors.