Add stdin option to command chains #190

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Right now, it's tedious to write command chains that require stdin input. For example:

chains:
  email-body:
    source: !request
      recipe: get-email
    selector: $.headers
  email-body-escaped:
    source: !command
      command: [sh, -c, "echo '{{chains.email-body}}' | jq -R -s '.'"]

The goal here is to reduce the number of use cases where users have to invoke the shell, without too much effort (see #153 for another example). This has two benefits:

• More user friendly
• Reduce cross-platform issues

Describe the solution you'd like
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A new field stdin on the !command variant:

chains:
  email-body:
    source: !request
      recipe: get-email
    selector: $.headers
  email-body-escaped:
    source: !command
      command: [jq, -R, -s, '.']
      stdin: "{{chains.email-body}}"

Describe alternatives you've considered
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The primary alternative is doing nothing, since this is already possible by invoking the shell directly.

Additional context
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If anyone wants to implement this, the work involved is:

• Add stdin field (type Template) to ChainSource::Command
• Add logic to render the stdin value, then pipe that to the subprocess in render.rs
• Add tests in template.rs
• Add docs in chain_source.md