Also remove the existing image, thereby removing the filename expansion
example and the file-existence highlighting example.

- Move an entry to another section.
- Fix commit hash width.
- Add missing punctuation.

That revision was itself a followup to 51614ca2c9.

Fixes #316.

* danielsh/i316-v2:
  'main': Enable fallback to the 'arg0' style.
  'main': Set fallback style for the 'arg0' style.

The fallback style name uses '_' in anticipation of a future auto-fallback
feature keyed on style names.

'arg0' was previously known as 'commandtypefromthefuture'.

* upstream/pr/374:
  'main': Directly count spaces to skip, don't leverage proc_buf length

Main highlighter run on itself, on the optimized version:

- optimized (8 runs, 3 last noted):
1.1201650000
1.1074430000
1.1263810000

- unoptimized (8 runs, 3 last noted):
1.5746400000
1.5115250000
1.5155440000

Average difference: 0.415907

Part of issue #139.

Fixes #344.

Fixes #96.

Running main highlighter on itself (both runs are on the optimized
version):

- clean (8 runs, last 3 noted):
1.7007670000
1.7330720000
1.7038810000

- optimized (8 runs, last 3 noted):
1.5007230000
1.5142960000
1.4973320000

Average difference: 0.208456

When parsing main-highlighter with itself, the *_check_path function is
called 426 times. Note that there are 686 region_highlight resulting
entries.

Fixes #335.

Fixes #333.

Fixes #368.

Fixes a subset of issue #237.

The heretofore code warned once per keypress.