Cider

- Fixed installation issue with click dependency.

- Added YAML support for bootstrap and defaults files.
- Fixed issue with setting icons for local paths.
- `brew upgrade` is now used instead of install when restoring if a formula
is outdated.
- Fixed an exception when restoring on Python 3.
- Fixed an error on old compilers without support for subscripting in
Objective-C.

- Fixed broken execution flow for `--version` flag.
- Fixed false-positive for xcode-select requirement.

- Added support for setting environment in bootstrap.
- Added option to skip prompts about installation failures during restore.
- Removed requirement for Xcode when restoring.

- Added `cider tap missing` command.
- Allow adding multiple items at once using `addlink`.

- Fixed bug where Homebrew's debug output wasn't getting ignored in
`brew missing`.
- The bootstrap file and cider directory are now created automatically if
they do not exist.
- The location of the cider directory can now be changed according to the
XDG basedir spec.
- Cider's support directory is now located in ~/Library/Application Support
by default instead of ~/.cider/.cache.
- Added the `addlink` & `unlink` commands.