Fix bash-it preview

[NoahGorny]

• lib: preview: Load only bash-it.sh when previewing
• lib: search: Increase delay in _bash-it-flash-term to 0.2 secs

Hi @gaelicWizard, seems like I bumped into a problem with bash-it preview. Please take a look!

[gaelicWizard]

Both of these changes look good to me.

(And I think I just outed myself as not having a theme enabled by default!)

[gaelicWizard]

I'm thinking of rewriting _bash-it-flash-term(). I'd like it to be a generic print-and-flash. (Right now, it assumes the term has been printed once already and expects the length passed in.)