Improve backup system

This commit introduces several improvements to the backup system.

* Backups are made every 8 seconds for buffers that have been modified
  since the last backup.

* The `permbackup` option allows users to specify that backups should
  be kept permanently.

* `The backupdir` option allows users to store backups in a custom
   directory.

Fixes #1641
Fixes #1536
Ref #1539 (removes possibility of race condition for backups)

internal/config/settings.go

@@ -247,6 +247,7 @@ var defaultCommonSettings = map[string]interface{}{
 	"autoindent":     true,
 	"autosu":         false,
 	"backup":         true,
+	"backupdir":      "",
 	"basename":       false,
 	"colorcolumn":    float64(0),
 	"cursorline":     true,
@@ -261,6 +262,7 @@ var defaultCommonSettings = map[string]interface{}{
 	"keepautoindent": false,
 	"matchbrace":     true,
 	"mkparents":      false,
+	"permbackup":     false,
 	"readonly":       false,
 	"rmtrailingws":   false,
 	"ruler":          true,