add ./bin to path if present (for spring)

.zshrc

DEFAULT_PATH=$PATH

# ...

# add ./bin to path for Spring (rails)
add_bin_to_path_if_necessary() {
  if [[ -d ./bin ]]; then
    export PATH=./bin:$DEFAULT_PATH
  else
    export PATH=$DEFAULT_PATH
  fi
}

# These get run before every command
function precmd() {
  add_bin_to_path_if_necessary
}