backup.sh v0.1.0

Nearly a complete rewrite of all supporting logic and configuration file parsing.

Configuration changes:

• there is a single toplevel configuration file called Backupfile
• jobs may share the toplevel configuration file or be placed in separate files that are referenced from the toplevel
• jobs are referred to by name, not by their configuration file path
• configuration of all job's aspects (prune, schedule) is now unified
• configuration files are loaded in a clean, separate subshell
• well-defined functions for loading or locating other files are provided in the configuration file environment

CLI changes:

• there is a single entrypoint script called backup.sh or bsh
• all operations with bsh follow the same syntax of bsh <VERB> [JOB [ARGS...]]
  (except for bsh unmount which takes ARGS but not JOB)

Packaging changes:

• a PKGBUILD is provided (installation is open-coded, no Makefile yet)