Clarify how to specify UMD export name

[msfragala]

What this does

• Adds a name option to the CLI, which accepts a string.
• Briefly discusses options for specifying UMB export names in the README

Context

When bundling UMD build, it is often useful to control the export name. For instance, microbundle defaults to a snake_case version of the name field in package.json, but camelCase is generally preferred in JavaScript. Trying do so myself, I noticed the line that determines the name feeds from two additional sources:

• options.name which seems like it comes from the CLI options, but no such options exists
• pkg.amdName which is not a standard field AFAIK or mentioned in the README

This PR intends to fix both those things. 🙂

It's worth noting that adding --name from the CLI already feeds to options.name. This just makes that fact official, rather than implicit.

[developit]

Beautiful!

[developit]

FWIW I would like to also switch the default name generation to use camelCase instead of snake_case.