Add cross-env support

[BhaveshSGupta]

To make build possible even on windows.

[jaller94]

This works for you?
I thought lines with multiple variables and commands need one cross-env-shell at the start of the line instead of multiple cross-env.
https://www.npmjs.com/package/cross-env#cross-env-vs-cross-env-shell

Don't have a Windows to test this though. If you say this works, it can get merged.

[BhaveshSGupta]

cross-env works fine,
Although I had checked only make-dev and make-watch. But I found make is giving error at APP_VERSION=git tag --points-at HEAD saying tag is not defined.

kentcdodds/cross-env#206

[jaller94]

Without a tag on the currently checked out HEAD, the command should not output a string.

[jaller94]

I don't see a way to edit your branch so I "forked" your PR to #89.

[BhaveshSGupta]

@jaller94 This issue is now fixed as per my new commits, can you review them.

[jaller94]

Fixes #95.

[jaller94]

For every release after v0.2.11 Windows users will have to set their NPM shell to power-shell.
We start using string substitution to fix issue #131.

Further information:
https://www.npmjs.com/package/cross-env#windows-issues