That means we can now have highlighters active at global, buffer, and
window scope. The add-highlighter and remove-highlighter syntax changed
to take the parent path (scope/group/...) as a mandatory argument,
superseeding the previous -group switch.
Sometimes the implementation of `man` will display errors, e.g.
```
<standard input>:4808: warning [p 54, 13.2i]: can't break line
```
Those errors are harmless but are still reported on the debug buffer,
so we hide them by redirecting the standard error stream to /dev/null.
Looking up the man page for `index` was failing on systems using
GNU/coreutils. The `:man` command matched whatever page it was given with
the `expr` utility. This tool behaves as expected when it follows strictly
the POSIX standard but the GNU implementation introduces additional commands
(including `index`), about which the standard states:
```
The use of string arguments length, substr, index, or match produces unspecified results.
```
As a result, parsing the man page number is now implemented with pure
shell expansions, to avoid triggering an undefined behavior when the topic
searched is one of the keywords above.
Level out the builtin commands loaded at startup in terms of format and
expressiveness. The following convention was followed:
* commands that take more than one argument have to be described along
with their parameters prior to the actual documentation, otherwise the
docstring consists in a capitalized sentence
e.g. `command <arg1>: do something`
* optional arguments are enclosed in square brackets, to comply with the
format used for hardcoded commands
e.g. `cd [<directory>]`
* describe the effects of the command in the documentation string and
omit implementation details unless they are relevant. Usually command
names include the name of the tool they use, so they don't need to be
redundantly mentioned
e.g. `tmux-new-pane <arguments>: open a new pane`
* document the format the parameters to the commands, or list them if
they are to be chosen among a list of static values (c.f. `spell.kak`)
