Command and provisions for installing lilypond

[noteflakes]

In order to make installation easier, and also in order to be able to use a relatively up to date version of lilypond, lydown should be able to install its own copy of lilypond, which it should download (the binary that is) and install somewhere.

Mode of operation:

• When lydown is invoked (for compilation), it checks that lilypond is installed, and verifies its version. If lilypond is not installed or too old, lydown should display an error message and suggest installing lilypond by issuing a command such as lydown install lilypond, and then exit.
• When lydown is invoked with the version command (lydown version), it should display the lydown version as well as the lilypond version. If lilypond is not available or too old, it should display the error message and suggest installing lilypond as above.
• When the install command is invoked, lydown performs the following:
  • detect the platform
  • download the relevant archive from the lilypond website
  • extract its content (according to platform), then put it in /usr/local/lydown/lilypond/<version>/
  • create a symlink at /usr/local/bin/lilypond
  • test that lilypond works by detecting its version

Remarks:

• Windows not supported at first, only linux and osx
• Should eventually add commands to uninstall, list versions etc

[noteflakes]

Actually, better put files in ~/.lydown/packages. That way we don't need to deal with permissions.