proposal: move cmd example tools to external repository (to reduce external dependencies of flac)

[mewmew]

This is a proposal to move all tools of the cmd/ subdirectory to a new/external repository (e.g. mewkiz/flac-tools). The main motivation is to reduce the need for external dependencies from the flac core libraries.

Moving cmd/ out from the flac repository would help to remove the following dependencies from the mewkiz/flac Go module:

• github.com/go-audio/audio (only used by cmd/flac2wav and cmd/wav2flac)
• github.com/go-audio/wav (only used by cmd/flac2wav and cmd/wav2flac)

Similarly, #22 is aimed at reducing external dependencies.

An alternative approach to this proposal may be to create additional go.mod files for the cmd/ subdirectory, and thus allowing the mewkiz/flac repository to contain multiple Go modules (each tracking their dedicated dependencies).

CC: @karlek