Feature: Highlight non-printable characters

Adds a new `-A`/`--show-all` option (in analogy to GNU Linux `cat`s option) that
highlights non-printable characters like space, tab or newline.

This works in two steps:
- **Preprocessing**: replace space by `•`, replace tab by `├──┤`, replace
newline by `␤`, etc.
- **Highlighting**: Use a newly written Sublime syntax to highlight
these special symbols.

Note: This feature is not technically a drop-in replacement for GNU `cat`s
`--show-all` but it has the same purpose.

assets/syntaxes/show-nonprintable.sublime-syntax

@@ -0,0 +1,25 @@
+%YAML 1.2
+---
+# http://www.sublimetext.com/docs/3/syntax.html
+name: Highlight non-printables
+file_extensions:
+    - show-nonprintable
+scope: whitespace
+contexts:
+  main:
+    - match: "•"
+      scope: support.function.show-nonprintable.space
+    - match: "├─*┤"
+      scope: constant.character.escape.show-nonprintable.tab
+    - match: "␤"
+      scope: keyword.operator.show-nonprintable.newline
+    - match: "␍"
+      scope: string.show-nonprintable.carriage-return
+    - match: "␀"
+      scope: entity.other.attribute-name.show-nonprintable.null
+    - match: "␇"
+      scope: entity.other.attribute-name.show-nonprintable.bell
+    - match: "␛"
+      scope: entity.other.attribute-name.show-nonprintable.escape
+    - match: "␈"
+      scope: entity.other.attribute-name.show-nonprintable.backspace