package.json: bump version 0.2.10 -> 0.2.11

New features:
- The firenvim browser button now contains a link to the troubleshooting
  guide (eb3abef)

Bug fixes:
- Firenvim will work on websites with strict CSPs on Firefox (50aae17, fa6406d)
- Neovim plugin has been fixed after neovim breaking change (35caecb)
- Unnecessary files aren't shipped on thunderbird anymore (f62386a)
- firenvim#install() will attempt to recover from cmd.exe and
  powershell.exe not existing in the PATH on windows (35ddc81)
- The thunderbird compose window will stop closing itself after some
  time without any activity (553c913)
- Negative or null-dimensions characters do not break the renderer
  anymore (3624c47)