Document bankai build options

[tnguyen14]

I just stumbled into 2 options for the build command that were really critical for me, but I didn't see them documented anywhere. I had to search through the issues and pull requests to find them.

bankai build index.js docs # output dir into a different path than the default "dist"
bankai build index.js --base https://domain.com/sub-path # prepend a base href to scripts/ styles URL

I was going to create a PR to add these, but I am not sure how these options should be handled, as they're only specific to the build commands. The current docs show options that are applicable to the other commands as well.

Please let me know. More than happy to help with this.

[goto-bus-stop]

Hey thanks! Maybe a bankai build section nested inside the Usage section in the readme would be good? And a bankai --help build command similar to how bankai --help config works?