Better flag name for output directory than --dir

[jimafisk]

The "--dir" flag in the build/serve commands is just used to change the name of the directory that holds that built site. Currently this defaults to "public" but you can set it to whatever you want (some CI builds might require a specific name). Maybe there's a better name for this flag to make it more clear. I've received feedback from @brikou who initially thought this was for running plenti commands outside of the project root, which is not the case.

Options for different flag name:

• --output
• --static

[jimafisk]

This flag has been changed to --output in v0.4.0+