Default should accept boolean values

[cocoabythefire]

I'd like to be able to initialize a boolean field with a default value of true or false but it currently only allows numbers or strings for defaults.

So I would like to be able to write:

table.bool('myProperty').default(true);

[wbyoung]

This does seem like a valid case to handle. Without trying anything, my only concern would be surrounding comparability and whether all databases can handle a bool directly or if they can all handle a default of 1/0 for a boolean column.

I haven't been all that active on this project recently, but a PR would be welcome. 😄