Allow triggers to reference session state with `\(variable)` syntax.

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• iTerm2 version: Build 3.1.beta.6
• OS version: macOS Sierra 10.12.6
• com.googlecode.iterm2.plist

Detailed steps to reproduce the problem:

1. Go to some arbitrary directory that is not the root directory.
2. Generate terminal output that activates the trigger. In this case, a relative file path for a file that exists relative to the current directory, followed by a semicolon, a line number, another semicolon, and a column number. For example, test/rules-parsing/no-restricted-imports.test.js:4:20
3. Double click the relevant captured output list item. This should open up the Atom text editor with that file path and line number/column number combo.

What happened: Atom opens but it doesn't load the right file. It instead creates a new blank file.

What should have happened: Atom should have opened with the right file.

It seems the trigger has no sense of what the current directory is and only makes the root directory available to its parameter command. The trigger also seems to run the parameter command in an sh subshell but it would be nice if that could be configurable. Personally, I would rather use zsh.

changed milestone to %Review for next version

This is a deeper issue than just captured output. Triggers in general have no way to refer to any of the session's state. Smart selection actions do, but it's counterintuitive. I have a good syntax for state that badges use, and I need to deprecate the various other forms and make that universal.

changed title from Trigger + Captured Output: How to use the current directory? to {+Allow triggers to reference session state with \(variable) syntax.+}

mentioned in issue #2219