Implement configurable detach key

Implement configurable detach keys (for `attach`, exec`, `run` and
`start`) using the client-side configuration

- Adds a `--detach-keys` flag to `attach`, `exec`, `run` and `start`
  commands.
- Adds a new configuration field (in `~/.docker/config.json`) to
  configure the default escape keys for docker client.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

term/ascii.go

@@ -0,0 +1,66 @@
+package term
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ASCII list the possible supported ASCII key sequence
+var ASCII = []string{
+	"ctrl-@",
+	"ctrl-a",
+	"ctrl-b",
+	"ctrl-c",
+	"ctrl-d",
+	"ctrl-e",
+	"ctrl-f",
+	"ctrl-g",
+	"ctrl-h",
+	"ctrl-i",
+	"ctrl-j",
+	"ctrl-k",
+	"ctrl-l",
+	"ctrl-m",
+	"ctrl-n",
+	"ctrl-o",
+	"ctrl-p",
+	"ctrl-q",
+	"ctrl-r",
+	"ctrl-s",
+	"ctrl-t",
+	"ctrl-u",
+	"ctrl-v",
+	"ctrl-w",
+	"ctrl-x",
+	"ctrl-y",
+	"ctrl-z",
+	"ctrl-[",
+	"ctrl-\\",
+	"ctrl-]",
+	"ctrl-^",
+	"ctrl-_",
+}
+
+// ToBytes converts a string representing a suite of key-sequence to the corresponding ASCII code.
+func ToBytes(keys string) ([]byte, error) {
+	codes := []byte{}
+next:
+	for _, key := range strings.Split(keys, ",") {
+		if len(key) != 1 {
+			for code, ctrl := range ASCII {
+				if ctrl == key {
+					codes = append(codes, byte(code))
+					continue next
+				}
+			}
+			if key == "DEL" {
+				codes = append(codes, 127)
+			} else {
+				return nil, fmt.Errorf("Unknown character: '%s'", key)
+			}
+		} else {
+			codes = append(codes, byte(key[0]))
+		}
+	}
+	return codes, nil
+}