Add hstr as an external module

[sorin-ionescu]

hstr allows one to easily navigate history.

[zkanda]

hmm never tried hstr, but fzf is a good alternative as well.

[sorin-ionescu]

fzf is universal. hstr is specialized in history. I haven't used either. I know not which one is better for history.

[srijanshetty]

Considering closing this one as we aren't adding any new modules to core. @belak what's your opinion?

[belak]

I've considered closing this in the past, but as @sorin-ionescu opened it, I've left it alone.