syntax/justfile: support command annotation, variable substitutions and captured outputs

[pbsds]

• support -@! command prefixes
• support {{variablesubstitution}}
• support `captured shell commands`
• support ```multiline captured shell commands```

[Andriamanitra]

I also have a PR open for improving justfile syntax which addresses these issues (and more): https://github.com/zyedidia/micro/pull/3018

[pbsds]

LGTM