Coloring output discards command color information

To reproduce compare `crystal spec`, `crystal build`, etc running within and without guardian.
* Without guardian errors are red, `pending` has it's own easily skipped color, etc.
* Within guardian the entire command output is grey and to my impaired vision almost unreadable.

I suggest replacing
```
output = `#{command}`
output.lines.each do |line|
puts " #{line.gsub(/\n$/, "").colorize(:dark_gray)}"
end
```
With:
```
system command
```

`system` solves another problem with long running commands not showing output until exit with the current code.