feat: set environment variables via a flag, custom task names

[tommy-mitchell]

Closes #4. Closes #6.

• Allows setting environment variables through the --environment variable (aliases: --env, -e)
  • Variables can be given values in the form VAR:value
  • Passing a variable without a value (VAR) sets it to "true"
• Allows naming tasks via a prefixed name in the form name::command
  • Multi-word names must be surrounded by quotes
• Appends : running "<command>"... to task titles
  • Previously, the title was changed to Running "<command>"...
• Changes some internal formatting, uses snapshots for CLI tests, and removes comments from build

[tommy-mitchell]

After merging, create a reminder issue to use duplicate named capturing groups in the future: https://github.com/tc39/proposal-duplicate-named-capturing-groups