Add savehistory option

When savehistory is enabled, micro will save your command history across
sessions. This includes command-mode, shell-mode, open, jump-to-line...
Anything that uses up-arrow for history in the infobar.

This option is on by default.

Closes #874

cmd/micro/messenger.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bufio"
 	"bytes"
+	"encoding/gob"
 	"fmt"
 	"os"
 	"strconv"
@@ -485,6 +486,55 @@ func (m *Messenger) Display() {
 	}
 }
 
+// LoadHistory attempts to load user history from configDir/buffers/history
+// into the history map
+// The savehistory option must be on
+func (m *Messenger) LoadHistory() {
+	if GetGlobalOption("savehistory").(bool) {
+		file, err := os.Open(configDir + "/buffers/history")
+		var decodedMap map[string][]string
+		if err == nil {
+			decoder := gob.NewDecoder(file)
+			err = decoder.Decode(&decodedMap)
+			file.Close()
+		}
+
+		if err != nil {
+			m.Error("Error loading history:", err)
+			return
+		}
+
+		m.history = decodedMap
+	} else {
+		m.history = make(map[string][]string)
+	}
+}
+
+// SaveHistory saves the user's command history to configDir/buffers/history
+// only if the savehistory option is on
+func (m *Messenger) SaveHistory() {
+	if GetGlobalOption("savehistory").(bool) {
+		// Don't save history past 100
+		for k, v := range m.history {
+			if len(v) > 100 {
+				m.history[k] = v[0:100]
+			}
+		}
+
+		file, err := os.Create(configDir + "/buffers/history")
+		if err == nil {
+			encoder := gob.NewEncoder(file)
+
+			err = encoder.Encode(m.history)
+			if err != nil {
+				m.Error("Error saving history:", err)
+				return
+			}
+			file.Close()
+		}
+	}
+}
+
 // A GutterMessage is a message displayed on the side of the editor
 type GutterMessage struct {
 	lineNum int