Append instead of overriding trap handler (fixes #6)

[dcsobral]

Also fixes a missing triple quote on README.

Introduces OSHT_TRAP_OVERRIDE to revert to previous behavior, as a way to avoid pre-existing trap handlers that exit. I'd normally prepend instead of append to avoid that issue, but OSHT trap handler calls exit, which prevents other handlers from running.

[gaspardpetit]

When I tried this on my end, bash -c "echo $(trap -p INT TERM EXIT)" returns nothing, so trap -- "${3}${3:+;}${HANDLER}" ${SIGNALS} then fails with osht.sh: line 108: 3: unbound variable

I imagine trap -p is supposed to return something?