Stick with the following style:
* Header 1 with the module name is in `lowercase`.
* Other headers are in `Sentence case`. Common header names that should
be consistently used are `Aliases`, `Contributing`, `Functions`,
`Settings`, and `Zsh options`.
* The names `Zim` and `Zsh` always appear capitalized, even in the
middle of sentences.
* Prefer
print 'code indented with 4 spaces'
instead of
```zsh
print 'code fenced by lines with three back-ticks'
print 'unless you want syntax highlighting'
```
This change allow arbitrary Zim location path by setting a new
environment variable, ZIM_HOME. If the user does not set it, Zim falls
back to the old "${ZDOTDIR:-${HOME}}/.zim" location.
Closes#203
