Add tf-summarize integration & skipping tf init / cleanup

[blufor]

I've decided to add few features. The crucial one is adding two settings:

• skip_init
• skip_cleanup

These two work together with s3-cache plugin, so when using separate pipelines you don't have to re-init the whole thing.

The other part is adding tf-summarize integration so that plans can be formatted nicely.

[blufor]

Also contains solution to #139 now.

The container is testable (amd64 only) with these docker images:

• blufor/drone-terraform:8.4-1.3.1
• blufor/drone-terraform:latest

[blufor]

...and an example of real-world usage