Allow personal overrides to psqlrc

I wanted to add my own customizations after reading @mike-burns' post on
`psqlrc`, but my brief research indicates that there's not a clean way to source
a file only after checking to see if the file exists from psql. I think the best solution 
is to include a blank `psqlrc.local` file so that it can always be sourced, but for 
users with no overrides it supplies no settings.

Adding a blank `psqlrc.local` file that users can override with their own custom
dotfiles, which will allow users to override the thoughtbot-provided defaults
(while continuing to receive upstream changes) and make their own
personalizations.