Add zdebuglog for basic debug logging and profiling

[belak]

Proposed Changes

• Add a zdebuglog function which only prints if :prezto:debug is enabled.
• Add some basic startup logging to zmodload

Reason

This came about so I could work on debugging slow startup times of some modules. There may be a better way - I'm open to improvements or suggestions.

Notes

I'd like comments from the other maintainers on this, since I feel like there's got to be a better way... but if we like this, we can merge it in and start using it elsewhere, since it lets us provide more useful debugging info, rather than just using set +x.