allow simple variable expansion in .vexrc
argument parser trivia
split out sys/os interaction into separate wrapper function
move some functionality behind an improved vexrc object interface
convert asserts into error barfs
be extra careful about catching FileNotFoundError etc.
have main write its errors to stderr instead of stdout
minor cleanups
some basic tests
basic functional tests using subprocesses
add README example on workon-like pattern
add shell prompt section to README
add bash prompt section
add zsh prompt section
add ksh prompt section
add bad tcsh prompt section
add fish prompt section
--shell-config option which dispatches to new shell_config.py
cleanups 3 in main
handle 2.7's lack of sys.stdout.buffer in emit_shell_config