Refactor shell determination and bootstrapping sequence
Tab completion for revise(), new compost() func

eval idiomatic tab completion scripts for zsh and bash: just source
c_extras.sh. This behavior may be selectively disabled by exporting:

COMPOSURE_TAB_COMPLETION=0

prior to sourcing c_extras.sh.

add new function, compost(), which deletes a composed function from your
composure repo. use with care!
Release 1.2.1