Don't use -i by default with Fish (see purcell#87)

Correct a function name in the README

Preserve "" env var values: don't map them to nil

See purcell#31

Don't break if $SHELL is unset at load-time

Fixes purcell#20

Add note about fish and tcsh to README

Append exec-directory to exec-path, as is conventional

Use -d in place of --login for tcsh (fixes purcell#7)

Explicitly double-quote args, so empty var values don't mess things up

Fixes purcell#6

Can now single-quote format argument to printf

Unambigiously escape the null character (fixes purcell#5)