Export ENV vars rather than sourcing a script

[evanthegrayt]

Sourcing ~/.cdcrc is unnecessary, as it just sets variables that could be exported from ~/.bashrc or ~/.zshrc. This would also be safer, avoiding potentially malicious code in a file.

Rather than checking for the existence of the file, just check for the needed variables (really, just CDC_DIRS).

TODO:

• Don't source a ~/.cdcrc file
• Check for existence of CDC_DIRS
• Remove -s option, which re-sources the file
• Update documentation to instruct users to export CDC_DIRS and other variables.