This PR sets the default value of `OMF_CONFIG` variable in
framework init.fish file. The variable can still be overridden by
the user by setting it on `~/.config/fish/config.fish file`.
This is in preparation to rewriting install script in plain fish,
which will ditch config template and stop replacing `config.fish`
contents in favor of just appending Oh My Fish startup.
It's hard to read the template as it was being created using `echo`
instead of being defined on a separate file. Also adding some comments
to improve the configuration file readability.
As @GlitchMr commented, fish 2.1 has not been released yet and even
then, the command . will only be deprecated, not removed.
This reverts commit 03833fda7b.
