More Makefile tweaks

The change in 08281333 means that the width of emoji are used. I think the issue https://github.com/unicode-rs/unicode-width/issues/4 means that the wrong width is being calculated for emoji, and there happens to be one in the example.

Fix TextCellContents cjk width

This makes it possible to use them in scripts. Also, I couldn’t find any other program returned a different error code! So it’s being changed to 0.

Fixed #180.

It looks like zsh doesn't have a pkg-config file so we can't use it for
ZSHDIR.

Fixes #179.

Fix a bunch of issues with the Makefile

Now everything’s customisable (in the future!) in its own place. This was the last thing to do for #65.

Fixes #134, a bug that showed symlinks incorrectly as broken, but only when the file was listed directly on the command-line *and* the file was in a different directory to the one exa was being run in.

I’m not sure why the old code used `String::new()`, but it doesn’t seem to affect anything.

* `$BASHDIR` and `$FISHDIR` had incorrect paths
* Fix `$FEATURES` to actually work
* Add `-f` to `rm` invocations to quiet missing file errors
* Just use `man` to preview the manpage
* Fix (again) `make` not rebuilding
* Add `make help` to view all of the commands and variables

exa doesn’t capture any failures — it’s not really worth it for such a short-lived task, and doing this results in a smaller binary.

Fixes #128.

Always rebuild with `make`

Improvements to the makefile

Because the link style and status are now both available to the function that picks the colour style, we can have it highlight broken links differently.

Fixes #131.

Provides shell completion and makes all the command-line options consistent and complete.

Fixes #150, which tracked shell completion.

The old option descriptions were all written at different times, and needed some consistency. This makes everything consistent between the help text, README, man page, and shell completions, and fixes some mistakes made when writing them.

This also adds the missing options to the man page, fixing #175.

Also, make it so the same line doesn’t get added to the .bashrc every time provisioning is run.