Add support for multiple <<includes(file)>> and embedded <<includes(file)>>

[Mongey]

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Changes

=======

• Add support for multiple <<includes(file)>> and embedded <<includes(file) >>

Rationale

=========

I see there are cucumber tests to assert this behaviour is not supported, but I would like it to work this way 😄
Usecase: I have a template file in my orb, and rather than embedded the contents directly in my command

 - run: echo 'my-extremely-large-template\nis here' > foo.tmpl

I want to have

- run: echo '<< include(template.tmpl) >>' > foo.tmpl

where template.tmpl is

my-extremely-large-template
is here

Considerations

==============
n/a

[codecov]

Codecov Report

Base: 32.77% // Head: 33.13% // Increases project coverage by +0.35% 🎉

Coverage data is based on head (60b2e04) compared to base (168582f).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 60b2e04 differs from pull request most recent head 4faf29b. Consider uploading reports for the commit 4faf29b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #737      +/-   ##
==========================================
+ Coverage   32.77%   33.13%   +0.35%     
==========================================
  Files          46       47       +1     
  Lines        5415     5489      +74     
==========================================
+ Hits         1775     1819      +44     
- Misses       3387     3402      +15     
- Partials      253      268      +15     

Impacted Files	Coverage Δ
process/process.go	100.00% <100.00%> (ø)
api/policy/policy.go	65.86% <0.00%> (-5.44%)	⬇️
cmd/policy/policy.go	77.38% <0.00%> (-1.37%)	⬇️

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