Tests: Add basic coverage of -h / shorthelp.

For a start, this is basically an adapted version of tests/t2100-help.sh.
FIX: Short help -h inaccessible when custom config is used.

When short-circuiting the option parsing, the OPTIND must be adapted, too, so that the getopts processing loop is quit correctly.
ENH: Handle -h, shorthelp and help when a Fatal Error happens.

The user may need the help to solve any fatal error that appears while todo.sh isn't properly set up. As the help actions do not depend on any setting that the fatal errors check, we can still invoke them.

Factor out dieWithHelp() and use that for printing the fatal errors.
Merge pull request #94 from inkarkat/fix-shorthelp-custom-config

Fix shorthelp custom config