Extend invisibles

This code does two things, first it adds spaces as an invisible rendered by a "·". Second, it adds a property "invisibles" in the .tm_properties file that allows for changing which invisibles are shown and what glyph is used for each.

Invisibles are '\t', '\n', and ' ', to turn one off, add ~[\t \n] to
the settings string. To set the glyph used for the invisible, add [\t
\n][glyph_to_use].

For example, I have been using:
invisibles = "~ \t┊"
This turns off spaces, uses a "┊" for tabs, and leaves newlines at their default.

This is public domain