Add github page

[fvictorio]

It would be nice to have a github page to convert from MNID to the network/address pair (ant the other way around). I did a very ugly proof of concept, you can check it in https://fvictorio.github.io/mnid/.

This PR is just to check if you like the idea and to discuss the minimum content that this should have. For example:

• Right now it just decodes, it doesn't encode.
• Add styles, explanatory text, etc
• The code is hideous (going back to vanilla JS makes you grateful for frameworks)
• I used webpack to build the bundle, so that this uses the module that it's in src/index.js, but it doesn't have a NPM build script.