Support changing template

[suzuki-shunsuke]

When -k isn't specified, the template default is used at the moment.
You can't change the template dynamically.

In this proposal, we support changing the template dynamically according to the command line arguments and executed command result.

Drawback of this idea

if condition may be broken unexpectedly.
For example, if: Command startsWith "gh issue create" doesn't match gh -R suzuki-shunsuke/github-comment issue create.

Example

e.g.

exec_if:
- if: Command startsWith "tfmigrate plan"
  template_key: tfmigrate-plan
exec:
  tfmigrate-plan:
  - when: true
    template: |
      ## {{template "status" .}} {{if .Vars.tfaction_target}}{{.Vars.tfaction_target}}: {{end}} tfmigrate plan
      {{template "link" .}}
      {{template "join_command" .}}
      {{template "hidden_combined_output" .}}