Use direnv for directory-specific shell config

[adarsh]

Reason for Change

• On a client project, some directory-specific aliases and ENVs were required.
• The proposed solution was a little aggressive, clobbering cd kind of heavily.
• This seemed a little cleaner.

Changes

• Use direnv to load .envrc files, should they exist.
• Add some files to the .gitignore_global to avoid committing them to VC.