Add command history and cycling

Aerc will keep track of the previous 1000 commands, which the user can
cycle through using the arrow keys while in the ex-line. Pressing up
will move backwards in history while pressing down will move forward.

lib/history.go

@@ -0,0 +1,13 @@
+package lib
+
+// History represents a list of elements ordered by time.
+type History interface {
+	// Add a new element to the history
+	Add(string)
+	// Get the next element in history
+	Next() string
+	// Get the previous element in history
+	Prev() string
+	// Reset the current location in history
+	Reset()
+}