v20: Vendor license files

Add functions to test if a file is likely to be legal notice and copies
those files, up through the root of a dependency to the vendor location.

Closes #245

v19

Fix conflict message

v17

Fix for v16 bug. All vcs list commands now produce paths relative to the root of the vcs.

v15: Do releases to github.

For those that want officially created binaries instead of requiring go
get as the only install method.

Partially Fixes #110 (does not cover security bits, which are considered
out of scope).