Allow installing via non-GitHub URLs

Fixes #64.
Improve `runFresh` test helper to dry up the tests
Rename fresh output files in specs for consistency
Add missing quotes
Update remaining tests to use the updated runFresh helper
Check for `:` instead of `@` for consistency
Merge pull request #67 from freshshell/non-github-install

Allow installing via non-GitHub URLs
Merge pull request #68 from freshshell/improve-tests

Improve `runFresh` test helper to dry up the tests
Improve error message when symlink fails

Fixes #57.
Add missing full stops
Improve error message for symlink permission errors
Merge pull request #69 from freshshell/symlink-error

Improve error message when symlink fails
Support running subcommands as distinct binaries.
Pass arguments to subcommands.
Improve use of BIN_NAME variable.
Improve check for subcommands.
Add `fresh commands` to drive completions
Add completions for subcommands
Add subcommand documentation to the README
Make use of `fresh commands` in `fresh help`