Support ETL '$include' directive

[smgallo]

Description

Support specification of an ETL $include directive that will allow the contents of a file to be included directly in a JSON configuration file as a JSON-encoded string. This works similarly to the RFC-6901 JSON Pointers that we already implement and allows variables to be used in the file reference.

For example, given the include file etl_sql.d/query.sql:

SELECT
  name,
  description
FROM jobs j
WHERE 1;

And the configuration file

{
    "sql": {
        "$include": "etl_sql.d/query.sql"
    }
}

Generate the following JSON:

{
    "sql": "SELECT\n  name,\n  description\nFROM jobs j\nWHERE 1;\n
}

Motivation and Context

Improve readability of included complex SQL statements and general reuse.

Tests performed

Manually tested style prior to pushing. I think that using push --force causes the "continuous-integration/travis-ci/push" tests to fail.

smgallo@bobbiedarin:xdmod [14:59:29]$ ~/src/xdmod-qa/travis/build.sh -s                                                                                    (etl-include-directive ⬇ ⬆ ◼)
Running only style tests
X11 forwarding request failed on channel 0
Comparing HEAD to upstream/xdmod8.1 (165d38cd0e1e13fa12c641e28586f5f229c9330a)
travis_fold:start:syntax
Running syntax tests...
travis_fold:end:syntax
Syntax tests succeeded!

travis_fold:start:style
Running style tests...
travis_fold:end:style
Style tests succeeded!

travis_fold:start:extra
Running extra tests...
travis_fold:end:extra
Extra tests succeeded!

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[plessbd]

I wish this was pretty printed, but I wont hold it up cause of that ;)