feat: add ability to specify word count

[bguiz]

What

• Current usage:
  • mnemonics
• New usage:
  • mnemonics 15
  • (plus, current usage retained)
• Input validation added

Why

• Add ability for users to specify the number of words they would like in their mnemonic, instead of always using the default value

Note

• The new functionality was put into src/utils.js, in anticipation of making it easier to write tests.
• Given that there are currently no tests, I figured that adding them, and adding a test runner, would be something for a separate PR.