Indent code if embedmd command is indented

[halvards]

This change means that:

• indented embedmd commands are no longer ignored; and
• the indent (any whitespace as defined by strings.TrimSpace) is prepended to each line of code.

Added an integration test that runs embedmd twice. This ensures that embedded code is not duplicated or otherwise altered on the second run.

Reason for change: I implemented this change because I had code that was part of steps in an ordered list.