Add Support for HTTPRoute URLRewrite Filter

[danehans]

Add support for the HTTPRoute URLRewrite filter.

[danehans]

cc: @AliceProxy

[danehans]

Note: The user guide added by #565 should be updated when this issue is fixed.

[Xunzhuo]

I would like to assign this

[danehans]

@Xunzhuo done.

[Xunzhuo]

This is planned to be implemented in v0.3.0-rc1, right? Not the v0.2.0 @danehans

[danehans]

I didn't include this issue for v0.2.0 since we're running out of time. If you can have a PR submitted by tomorrow, then it can be included in v0.2.0.

[Xunzhuo]

Let us do this in v0.3.0-rc1. :)