Support text annotations

[MattesGroeger]

Optionally add annotations to bookmarks with shortcut mi. This is useful for preparing a code review.

Tasks:

• Update bm.vim to support annotations
• Add :Annotate command
• When on line without bookmark open insert dialog and add bookmark
• When on line with bookmark open insert dialog
• When on line with bookmark and annotation open edit dialog
• Add shortcut mi for adding annotations (i for insert annotation)
• Add annotations to persistence layer
• Show annotation in quicklist instead of line content (if set)
• Show annotation when jumping to bookmark
• Different color for annotations (configurable)
• Document feature (Readme.md & bookmarks.txt)

[MattesGroeger]

Feature completed with 224da65