feat(cli): add --watch option to create a plugin zip automatically

[koba04]

I think it would be nice if kintone-plugin-packer had a --watch option, so I've added it.
Using this PR, developers no longer run kintone-plugin-packer command manually after each change.

It's very useful, isn't it?

In this PR, I've updated the interface of cli.js to return always a promise if an error occurred. Current implementation might throw errors so we have to care about a case throw errors using try-catch. I think returning an error as a rejected promise seems to be better than throw an error at the point of consistency. Even though it might be a breaking change.

[teppeis]

snyk reported a security issue of chokidar, but it doesn't affect this package.

paulmillr/chokidar#686
fsevents/fsevents#198

[koba04]

Thanks!