Don't output "echo:" in .echo files

[PRouzeau]

It is now possible to build files in batch mode with .echo extension containing echo() messages content.
That is useful to create specifications, BOM, lists, batches, etc. which can be selected with a variable
However, the "echo:" leading text is still part of the output, which is an annoyance.
So, for output in files, it will be useful to removes these prefixes, giving a much cleaner file.

[runsun]

Second. Or at least make a switch that whoever doesn't want it can turn it off.

[t-paul]

There might be an actual output window at some point, but there are no plans on changing echo() and/or add formatting to console output.