Add main entrypoint to package.json

[ThomasHickman]

At the moment, if you execute require("mprogress") in node, the error Error: Cannot find module 'mprogress' is produced. Adding a main entry in package.json will fix this

[slince]

+1

[andreasvirkus]

For any future googlers (since this repo seems to be unmaintained), import it directly from the build/ dir:

import MProgress from 'mprogress/build/js/mprogress.min.js'
import 'mprogress/build/js/mprogress.min.css'