Refactor moe-theme-select-color and random-color

When I used to call the follow to set my theme, it would never work
quite the same as if I directly called `moe-theme-select-color`
manually.

```elisp
(setq moe-theme-mode-line-color 'purple)
(powerline-moe-theme)
```

For example, the file completion of IDO would never change
from blue. To eleminate this, I refactored the code you were using in
`moe-theme-select-color` out into it's own function, which both
`moe-theme-select-color` and `moe-theme-random-color` now both call
rather than independantly recreating the same logic.