Improve the terminal filter

The new filter comes with these enhancements:

 - Processing is rune-wise rather than byte-wise;
   it assumes UTF-8 input and single-cell wide characters,
   but this condition should be /usually/ satisfied.
 - Unprocessed control characters are escaped, `cat -v` style.
 - A lot of escape sequences is at least recognised, if not processed.
 - Rudimentary preparation for efficient dynamic updates
   of task views, through Javascript.

We make terminal resets and screen clearing commands
flush all output and assume that the terminal has a new origin
for any later positioning commands.
This appears to work well enough with GRUB, at least.

The filter is now exposed through a command line option.

terminal_test.go

@@ -0,0 +1,44 @@
+package main
+
+import "testing"
+
+// TODO(p): Add a lot more test cases.
+var tests = []struct {
+	push, want string
+}{
+	{
+		"\x1bc\x1b[?7l\x1b[2J\x1b[0mSeaBIOS\r",
+		"SeaBIOS\n",
+	},
+}
+
+func TestTerminal(t *testing.T) {
+	for _, test := range tests {
+		tw := terminalWriter{}
+		if _, err := tw.Write([]byte(test.push)); err != nil {
+			t.Errorf("%#v: %s", test.push, err)
+			continue
+		}
+		have := string(tw.Serialize(0))
+		if have != test.want {
+			t.Errorf("%#v: %#v; want %#v", test.push, have, test.want)
+		}
+	}
+}
+
+func TestTerminalExploded(t *testing.T) {
+Loop:
+	for _, test := range tests {
+		tw := terminalWriter{}
+		for _, b := range []byte(test.push) {
+			if _, err := tw.Write([]byte{b}); err != nil {
+				t.Errorf("%#v: %s", test.push, err)
+				continue Loop
+			}
+		}
+		have := string(tw.Serialize(0))
+		if have != test.want {
+			t.Errorf("%#v: %#v; want %#v", test.push, have, test.want)
+		}
+	}
+}