Add line editing, history and tab completion to shell.

doc/user-guide

@@ -106,6 +106,9 @@ Despite that, it does have some key features. Here are the features that are
 supported:
 
 * Processes can be started in the usual Unix manner.
+* Tab-completion.
+* Line editing.
+* History.
 * Background tasks ('&').
 * Standard output redirection ('>').
 * Piping stdin from a task to stdin of another ('|').
@@ -125,6 +128,11 @@ supported:
 * Basic wildcards ('foo*.bar').
 * Clearing the screen (Control-L, '^L').
 * Deleting the last typed word (Control-W, '^W').
+* Deleting everything before the cursor (Control-U, '^U').
+* Deleting everything after the cursor (Control-K, '^K').
+* Go to start of line (Control-A, '^A', Home).
+* Go to end of line (Control-E, '^E', End).
+* Move between words (Control-arrows).
 
 These features are missing from the shell: