Numerous bug fixes and improvements

[sekedus]

• Add Development Guide
• Add: download button
• Add: clear token button
• Add: validation css
• Add: parcel --public-url flag to make it work from a sub-directory
• Add: default_expires_at parameter on token url
• Add: .zip file name with hyphen (-)
• Add: download main repository .zip without repository folder
• Fix: can't download private repo main directory
• Fix: CORS error only with token (#150)
• Fix: ⚠ emoji display as plain text
• Edit: rename readme.md → README.md (explanation)

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[fregante]

There are a lot of changes that aren't improvements and need to be reverted, but it's hard to see what actually changed since the formatting was changed as well, without any clear advantage (" -> '). It's best to send multiple PRs