Make path formatting configurable

[phux]

I love this prompt, but I was missing the option to use for example %~ for the path formatting.

PR adds a check for PURE_PROMPT_PATH_FORMATTING.
Now users can define their preferred path formatting via

export PURE_PROMPT_PATH_FORMATTING="%~"

for example.
If PURE_PROMPT_PATH_FORMATTING is not defined it sticks to the current default %c.

[DFurnes]

This is great, thanks! 🎉