[FEAT] `exec` should take a heading option

[goliatone]

We should be able to provide a label for an exec command:

$ lgr exec 'Show cmd files' -- ls ./cmd/envset 
[✔] Show cmd files: ls ./cmd/envset
 └─[I] environment
   [1] envset
   [2] main.go
   [3] main_test.go
   [4] metadata
   [5] rc
   [6] template
   [7] testdata