agenda --tsv and agendaupdate: handle control and escape characters consistently

[michaelmhoffman]

agenda --tsv replaces newlines in text to be printed with r'\n'. This is necessary to avoid totally breaking anything downstream when newlines occur in event properties. This is common for description.

agenda --tsv should also:

• replace tabs with r'\t'
• replace backslashes with r'\\'

Furthermore:

• agendaupdate should reverse these escapes

[michaelmhoffman]

The unicode_escape codec will do these replacements, but it will also replace other non-Latin-1 codepoints with escapes. That seems undesirable, so better to do our own replacement when producing output. I'm not sure that there are any disadvantages to using it when reading things in, though.