Rename `MasStoreSearch` as `ITunesSearchAppStoreSearcher`.
Rename `StoreSearchMock` as `MockAppStoreSearcher`.
Rename variables/parameters of the above types as `searcher`.
Fix 'App Store.app' mention in help.
Partial #585
Signed-off-by: Ross Goldberg <484615+rgoldberg@users.noreply.github.com>
homebrew allows using gnu coreutils on MacOS. Therefore we should
comply with posix standards
```bash
$ mas help | tail +3
tail: cannot open '+3' for reading: No such file or directory
$ type tail
tail is /usr/local/opt/coreutils/libexec/gnubin/tail
```
Switching to posix compliant `-n` syntax is compatible with UNIX and GNU tooling.
```bash
$ mas help | /usr/bin/tail -n +3 | wc -l
18
$ mas help | tail -n +3 | wc -l
18
```
