conf option for dotless repos

[rking]

A lot of people structure their repos like this:

https://github.com/kyrylo/dot

That is, they add zshrc instead of .zshrc.

I picture it maybe looking like this:

dots:

• repo: https://github.com/kyrylo/dot
  add_dots: true

Then it would special-case bin and anything with a leading . already, but
otherwise link kyrylo/dot/foo to ~/.foo.

One small puzzle is loop-dots. It could:

• Skip repos like this (that is, just not work)
• Always check both the dotted and undotted versions (that is, be slow)
• Parse the ~/.../conf (which would allow us to remove the env var hacks)
• (Do something else?)