Multiline prompt support

Fish shell is not supported because I do not know to support it: it is way too restrictive. Tcsh support required some research, but it is now ready. Depends on `stty size` output and `cut` command presence.

Tests are currently missing, and there are no multiline themes shipped.

Status:

- [x] Add multiline prompt support to powerline core.
- [x] Add multiline prompt support to lint checker.
- [x] Add necessary changes to zsh bindings.
- [x] Add necessary changes to bash bindings.
- [x] Add necessary changes to tcsh bindings.
- [ ] Add necessary changes to fish bindings or document support status.
- [x] Update documentation.
- [ ] Add tests for new functionality.
- [ ] \(subjectable) Add `powerline/config_files/themes/shell/default_multiline.json`.
- [ ] \(related) Add tests for new shell theme sanity.

Ref #462.