Motivation:

It would be very useful to set up a simple mirror that mirrors the
result of an HTTP query into a repository, just like IFTTT.

Modifications:

- Added support for the following two new URL schemes:
  - `http`
  - `https`
- Renamed `Mirror.remotePath` to `remoteSubpath` to better reflect what
  it actually does.
- Made `Mirror.remoteSubpath` nullable because HTTP-to-CD mirror doesn't
  need it.
- Added `HttpMirror` and `HttpMirrorTest`.

Result:

- A user can mirror a certain HTTP endpoint into a file in Central Dogma.
- Closes line#154

Future works:

- Add support for basic and token-based authentication
- Add documentation