enable variable related checks in jshint

I wrote a short program that highlights the following mistakes related to variables:
- variable shadowing
- creating a variable that's never used
- using a variable that hasn't been assigned a value

https://www.khanacademy.org/computer-programming/variable-mistakes/6677049979437056

I'm proposing that we turn on the following jshint options:
- [shadow](http://jshint.com/docs/options/#shadow)
- [unused](http://jshint.com/docs/options/#unused)

Unfortunately, jshint doesn't have an option to check for use of unassigned variables.

No file or test is named in the issue. Start by locating the repository's JSHint configuration and checking how linting is run. Done means the shadow and unused checks are enabled; JSHint cannot provide the requested unassigned-variable check.

Written by the indexing model from the issue text.