Add support for relative urls in html fields

[chrisrhymes]

Resolves #14

• Added an optional base argument for relative urls within fields that contain html.
• Added tests
• Updated readme

Testing

• Checkout branch and run composer install and composer test

[jordanade]

Wow, thanks for the update Chris! I still have an issue though — my relative links don't start with a slash. (I am using a <base> tag in my headers.) To determine the presence of a relative link it would be more correct to check that the link doesn't begin with a protocol (or at least doesn't begin with "http://" or "https://".

Cheers, Jordan