Encourage override of toBBox, compareMinX, compareMinY for CSP issues

Where the Content Security Policy (or other) disallows the use of eval and related functions (e.g. browser extensions), the `toBBox`, `compareMinX`, and `compareMinY` methods can be overridden.

This makes it so the `Function` constructor is only used if a custom format is supplied.  Default implementations of `toBBox`, `compareMinX`, and `compareMinY` are run otherwise.

This is an alternative to #12 (and doesn't break the current API).