*-theme: make themes switchable on the go

[utrack]

This creates two new commands: nano-theme-set-dark and nano-theme-set-light. Use

  (nano-theme-set-dark)
  (nano-faces)
  (nano-theme)

to switch variables internally and apply the new theme on the go. It preserves the old behaviour of enabling a theme via (require nano-theme-dark), too. My elisp is not very good (couldn't make it work with deftheme's custom commands), but it's a start for #26 :)

[mrzor]

Looks good to me

[rougier]

Nice, thank you!