#7 Add Bookmark Insertion/GoTo w/HotKey

[mkudra]

1) Add ability to insert a numbered Bookmark at BOL of current line;
2) Indicate presence of Bookmark by gutter icon, highlighting, or some other visual cue;
3) Include ability to return to any Bookmark by HotKey (e.g., Ctl3);
4) Include persistence of Bookmarks across editing sessions.