Added note and colour tagging functionality

[singe]

This adds the ability to set notes for certain bytes, and to mark bytes or ranges of bytes with custom colours. This is particularly useful when trying to understand a binary format.

These are interactive functions:

Esc-c - prompt to mark a byte or mark'ed range of bytes as one of three colours
Esc-o - add a custom note to a byte. It will be displayed in the status line when the cursor is on the byte
Esc-d - delete a note
Esc-g - display a note
Esc-s - write the colour and note markup to a tag file (more on that below)

Markup can be persisted to a simple tag file with the following format:
n:
c: <colour number 1-3>
colour number is 1 - cyan, 2 - magenta, 3 - yellow

For example, a tag file marking up the first two fields of an ELF file in colour and with notes would look like:

0x0 c: 1 4
0x0 n: ELF Magic Bytes
0x4 c: 2 1
0x4 n: 1 - 32bit 2 - 64bit

Tag files are written to the currently edited filename with ".tags" appended