More flexible data tiddlers

This will allow 3 kinds of multiline content in data tiddlers and indented comments:

1. continuation lines
2. trimmed multiline content
3. indented multiline content

Continuation lines are lines which follow a line ending with a backslash "\". Continuation lines are trimmed. Whitespace before the \ is not trimmed.

        1234 \
            5678

becomes "1234 5678"

        1234\
            5678

becomes "12345678"

Trimmed multiline content is denoted by lines starting with a ":" (after optional whitespace)

        multiline:1234
        :  5678
        :  9abc

becomes "1234\n5678\n9abc"

Indented multiline content is multiline content which starts with an empty line, which is ignored.

        multiline:
        :  1234
        :    5678
        :      9abc

becomes "  1234\n    5678\n      9abc"