Also moved the templates out of this repository, and into the
zimfw/install repo.
This is a second big change after introducing the plugin mechanism. This
makes installation and upgrading of Zim straightforward. Maybe the most
important aspect of having the script in a single file is not having to
manage "git repos inside git repos" (see #297), since the single file
exists by itself and is not version-controlled (with git).
I've implemented a two-stage sourcing of the file, so most of the file
is only sourced when needed (namely when calling `zimfw` with any action
other than `login-init`). The two-stage process is designed to avoid
compromising the startup speed, which is our top priority.
In an effort to help making the script maintainable, I've broken it into
small ERB templates. This also adds the ability to pre-process the Zsh
code with Ruby code. To build the script, use `make`.
