Allow overriding recipes
Allow overriding recipes

* New user option `straight-recipe-overrides' that stores information
  about overridden recipes. This can be set directly or manipulated
  programmatically using new user-facing function
  `straight-override-recipe'.
* New internal function `straight--get-overridden-recipe' to read
  `straight-recipe-overrides', which is now used in
  `straight-use-package'.

Closes #57.
Closes #39.

raxod502/straight.el,raxod502/straight.el