Add a dummy marker module to simplify postprocessing by wrapper scripts

A simple marker module just indicating a marker string (for example, `@email`) will simplify collaboration between i3status and filter scripts that may want to insert extra information at some designated position of the output (for example, by using sed substitution).

I could open a PR if you like the idea.

Start with issue #120 and inspect how i3status modules expose their output to wrapper scripts. Implement a dummy marker module that emits a configurable marker string such as `@email`, then verify that wrapper scripts can identify and substitute the designated position.

Written by the indexing model from the issue text.