feat: add multiple working dirs

[sgettys]

Closes #81
This adds the ability to specify multiple terraform plans based on different working directories. The mixin config can now specify a list of directories for the different working dirs. Then each step can specify the working dir for the run of that step.

Example mixin config

mixins:
  - terraform:
      workingDirs:
        - infra1
        - infra2

Example install step with working directory specified for the step:

install:
  - terraform:
      description: 'infra 1'
      workingDir: 'infra1'
      vars:
        infra1_var: ${bundle.parameters.infra1_var}
      outputs:
        - name: infra1_output
  - terraform:
      description: 'infra 2'
      workingDir: 'infra2'
      vars:
        infra2_var: ${bundle.parameters.infra2_var}
      outputs:
        - name: infra2_output

Updated integration test with a multiple working dirs bundle.
Added usage docs.

[sgettys]

Refactor to have the build step inspect all actions for "workingDir" specified and collect them at build time. Fallback to the "workingDir" set in the mixin config or default