Add tabmovement option

This option makes micro treat spaces at the beginning of lines as if
they are tabs. This option only does anything if tabstospaces is already
on. E.g. micro will move over 4 spaces at once when at the start of a
line.

Closes #616

cmd/micro/actions.go

@@ -111,7 +111,21 @@ func (v *View) CursorLeft(usePlugin bool) bool {
 		v.Cursor.Loc = v.Cursor.CurSelection[0]
 		v.Cursor.ResetSelection()
 	} else {
-		v.Cursor.Left()
+		tabstospaces := v.Buf.Settings["tabstospaces"].(bool)
+		tabmovement := v.Buf.Settings["tabmovement"].(bool)
+		if tabstospaces && tabmovement {
+			tabsize := int(v.Buf.Settings["tabsize"].(float64))
+			line := v.Buf.Line(v.Cursor.Y)
+			if v.Cursor.X-tabsize >= 0 && line[v.Cursor.X-tabsize:v.Cursor.X] == Spaces(tabsize) && IsStrWhitespace(line[0:v.Cursor.X-tabsize]) {
+				for i := 0; i < tabsize; i++ {
+					v.Cursor.Left()
+				}
+			} else {
+				v.Cursor.Left()
+			}
+		} else {
+			v.Cursor.Left()
+		}
 	}
 
 	if usePlugin {
@@ -130,7 +144,21 @@ func (v *View) CursorRight(usePlugin bool) bool {
 		v.Cursor.Loc = v.Cursor.CurSelection[1].Move(-1, v.Buf)
 		v.Cursor.ResetSelection()
 	} else {
-		v.Cursor.Right()
+		tabstospaces := v.Buf.Settings["tabstospaces"].(bool)
+		tabmovement := v.Buf.Settings["tabmovement"].(bool)
+		if tabstospaces && tabmovement {
+			tabsize := int(v.Buf.Settings["tabsize"].(float64))
+			line := v.Buf.Line(v.Cursor.Y)
+			if v.Cursor.X+tabsize < Count(line) && line[v.Cursor.X:v.Cursor.X+tabsize] == Spaces(tabsize) && IsStrWhitespace(line[0:v.Cursor.X]) {
+				for i := 0; i < tabsize; i++ {
+					v.Cursor.Right()
+				}
+			} else {
+				v.Cursor.Right()
+			}
+		} else {
+			v.Cursor.Right()
+		}
 	}
 
 	if usePlugin {

cmd/micro/settings.go

@@ -196,6 +196,7 @@ func DefaultGlobalSettings() map[string]interface{} {
 		"splitBottom":    true,
 		"statusline":     true,
 		"syntax":         true,
+		"tabmovement":    false,
 		"tabsize":        float64(4),
 		"tabstospaces":   false,
 		"termtitle":      false,
@@ -231,6 +232,7 @@ func DefaultLocalSettings() map[string]interface{} {
 		"splitBottom":    true,
 		"statusline":     true,
 		"syntax":         true,
+		"tabmovement":    false,
 		"tabsize":        float64(4),
 		"tabstospaces":   false,
 		"useprimary":     true,

cmd/micro/util.go

@@ -82,6 +82,16 @@ func IsWhitespace(c rune) bool {
 	return c == ' ' || c == '\t' || c == '\n'
 }
 
+// IsStrWhitespace returns true if the given string is all whitespace
+func IsStrWhitespace(str string) bool {
+	for _, c := range str {
+		if !IsWhitespace(c) {
+			return false
+		}
+	}
+	return true
+}
+
 // Contains returns whether or not a string array contains a given string
 func Contains(list []string, a string) bool {
 	for _, b := range list {

runtime/help/options.md

@@ -65,6 +65,11 @@ Here are the options that you can set:
 
 	default value: `off`
 
+* `tabmovement`: navigate spaces at the beginning of lines as if they are tabs (e.g. move over 4 spaces at once).
+   This option only does anything if `tabstospaces` is on.
+
+    default value: `off`
+
 * `autoindent`: when creating a new line use the same indentation as the 
    previous line