Resolve .ts files as well as .js files

[koddsson]

When you have a .ts file such as button.ts and try to import it without it's file extension, the current config will error with the message:

app.js
1:1 import/no-unresolved  (https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/no-unresolved.md)
	Unable to resolve path to module './button'.


✖ 1 problem (1 error, 0 warnings)

It's because by default the import plugin will only look for .js files.

This PR passes an array of file extensions for the import plugin to consider as settings in the es6 config.

Ref: https://github.com/benmosher/eslint-plugin-import/blob/master/README.md#importextensions