allow for checkboxes in the "requirements" for boolean install parameters

[seancoyne]

Allows for checkbox requirements. This makes it easier to ask "yes/no" type questions when installing a package. (I used this in the FarCry recipe I just sent a pull request (#16) for against the master branch if you want to see a working example)

[atuttle]

Excellent use of the type attribute. Exactly what I built it in for! :D