Instead of having use() report a normal error (which is actually fatal)
and returning >0, and calling use || exit $? everytime; just report the
fatal error and quit already

* move all backend code to separate functions
* call those functions directly
* simplify argument parsing
* remove redundant verbose begin/end messages

Introduce fatal(), error(), and info()

* refactor error logging code
* make all errors go to stderr
* make fatal errors actually fatal (exit instead of return)
* make output consistent

Forgot to set VCSH_COMMAND after the refactoring

github:
closes: #4

* Handle and sanitize all input in extra block
* Use exported variables instead of daisy-chaining them through functions
* Improve error detection and handling
* Re-order functions in alphabetical order

New feature:
* `vcsh <repo> <gitcommand>` is synonymous with `vcsh run <repo> git <gitcommand>

* Be less aggressive when asking for confirmation
* Work independently from $PWD

github:
fixes #6

for github:
fix #9

Do not rely on XDG settings being the defaults

The examples may now fail/act weirdly if XDG variables are not set, yet it's an overall benefit to change this.