Fixing issues #7 and #8

• Fix issue #7
• Fix Issue #8
• Update packages to fix security issues
• Switch to dotenvx and add the NODE_ENV environment variable in .env for debugging purposes.
• Few other modifications and optimisations

Switch to WEBP

Default image format is now WEBP, in order to reduce images' size.

Also package updates to address this security issue (see also here)

v1.0.0

• Switched from socket.io to WebSocket. (There is a socket-io branch but it relies on Socket.io v2.3.0 which is outdated and presents a security issue)
• cquant doesn't seem to be maintained anymore and is replaced by imagecolors. As a consequence, I had to switch from sharp to gm, although the latter is less effective than the former.
• Optimisation : instead of dealing with the cover of each track, images are cached on album base to reduce processing time.
• Minor graphical changes.