Move jshint config into a separate file

It seems better for clarity to store different things in different places.
package.json is already a big and complicated file, and .jshintrc is
a special file precisely for storing this configuration.

Not to mention that jshint authors themselves have switched away
from using jshintConfig key:
https://github.com/jshint/jshint/commit/91bf14ddc81821a54b711a06fe229f0b2427c8e6