pretty logging for drone exec

[jmccann]

Basically stole and modified https://github.com/cncd/pipeline/blob/master/pipeline/rpc/line.go#L49 to make drone exec pretty again like it was in Drone 0.5

Example .drone.yml

workspace:
  base: /drone

pipeline:
  first:
    image: ubuntu:16.04
    commands:
      - touch /drone/helloworld
      - sleep 10
      - echo counted?

  second:
    image: ubuntu:16.04
    commands:
      - echo "hello"
      - ls /drone/helloworld

Example output with changes:

$ ./drone exec
[first:L0:0s] + touch /drone/helloworld
[first:L1:0s] + sleep 10
[first:L2:10s] + echo counted?
[first:L3:10s] counted?
[second:L0:0s] + echo "hello"
[second:L1:0s] hello
[second:L2:0s] + ls /drone/helloworld
[second:L3:0s] ls: cannot access '/drone/helloworld': No such file or directory
2017/06/28 14:23:33 drone_step_1 : exit code 2