Avoid matching on single quotes

[ultraflynn]

The first group match in the regexp would match on a single quote and that resulted in results like this:

it's a nice day would become It'S A Nice Day

I have expanded the \w shortcut into the full version and added the single quote char: [^a-zA-Z0-9_']

I added a test to the README to cover this as well.

[grncdr]

Thanks @ultraflynn: this is now released (along with international character support) in capitalize@1.0.0.

[ultraflynn]

Marvellous. Thanks for that.