Watcher support for mock

[jknight]

Feature request:

Add --watch support for mock the way html has it:

docker run -p 8080:8080/tcp -it --rm -v $PWD:/doc quay.io/bukalapak/snowboard html --watch api.apid

Apologies if this is already in there but the equivalent with mock instead of html:

docker run -p 8080:8080/tcp -it --rm -v $PWD:/doc quay.io/bukalapak/snowboard mock --watch api.apid

gives:

✖ Parsing input(s)... error
Error: ENOENT: no such file or directory, open '--watch'