Add rainbow (elite text) attribute feature

New key "rainbow" in ctrl/attr.ini
New Ctrl-A codes: 'X' to turn on repeating/wrapping rainbow attributes,
 'x' to turn on non-repeating/wrapping rainbow attributes.
New @-code: RAINBOW:x to set the list of (comma-separated) rainbow attribute
 values (in same form as attr.ini).
 When used in a display file, the rainbow attribute change is temporary.

Ever wanted to add alternating attributes to @-code expanded text in display
files? Now you can.

Also can be used to easily/quickly create display files with repeating
elements (e.g. commands/options) using alternating attributes.