Support pre_commands in .drone.yml

[davidhiendl]

Support pre_commands in .drone.yml with same syntax as commands and prepend pre_commands to commands with convert plugin.

Example:

kind: pipeline
name: default

steps:
  - name: test
    image: alpine
    pre_commands:
    - echo "abc"
    commands:
    - echo "bcd"

And transform it into:
Example:

kind: pipeline
name: default

steps:
  - name: test
    image: alpine
    commands:
    - echo "abc"
    - echo "bcd"