add default prefix and allow prefixes with spaces

This is a change I would like peer-reviewed. If the default strategy changes are rejected, I can submit a separate request to just allow prefixes with spaces.
Allows you to use the default strategy (match key attributes) when a '=' is in the locator.
Locator `page?a=b` will still cause error, but can be changed to `default=page?a=b`.
Locator prefixes will now be space-insensitive so the following will work when the previously did not:
* `partiallink=text`
* `x path = //div`

Fixes #193