Auto cd when selecting file from MRU

[LamprosPitsillos]

Fixing #52

It is off by default, as to not cause any breaking changes.

If you want to use this just do :

local theme = require("alpha.themes.theta") --or require("alpha.themes.startify")
theme.mru_opts.autocd = true
alpha.setup(theme.config)

[goolord]

ty for your contribution!