Add support for underscore separator in numbers

[Victor4X]

Closes #7

If you want this implemented in a different way, feel free to tell me how :)

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[Victor4X]

Something must've changed between your last deploy and today, as netlify's deploy now fails with this error:

Failed to compile.
./src/Banner.js
  Line 222:21:  Using target="_blank" without rel="noopener noreferrer" is a security risk: see https://mathiasbynens.github.io/rel-noopener  react/jsx-no-target-blank
​
────────────────────────────────────────────────────────────────
  "build.command" failed                                        
────────────────────────────────────────────────────────────────

I'm not sure if you want this fixed in a separate PR, but for now I will add a commit to this one.