Improve error message and documentation related to 'options.prefix'

[msssk]

README.md: add documentation for options.prefix
index.ts: add error message if options.prefix is not specified with options.main

Fixes #121

[msssk]

The logic on line 358 assumes that options.prefix is set.

• if we change options.prefix to empty string we can prevent the error, but the module still isn't located
• if we default options.prefix to options.name then the module is located, but output is incorrect
• always defaulting options.prefix to options.name seems error-prone, so the best approach may just be enforcing explicit usage of options.prefix

[vytenisu]

Would be worth checking if it works with -name, -prefix and --main specified all together when name and prefix do not match. Not sure if this would work.

On the other hand this improves situation a lot. Thank you.