(feat) Variables from comments

[zellers]

This pull request implements functionality to define variables for parameter replacement as comments above the query.

This fixes the remaining part of issue #744.

Syntax follows proposed format in #744 and mirrors existing syntax for defining connection to be used as comment above the query.

-- @var key = value

Example:

-- @var FOO = "bar"

SELECT :FOO as baz

• Your code builds clean without any errors or warnings
• You have made the needed changes to the docs
• You have written a description of what is the purpose of this pull request above

[gjsjohnmurray]

Thanks for your contribution. I have suggested a minor change to the documentation.