Enables using variables and functions as arguments

This change an extra level on indirection for two cases:

+ when an association or an alist is required, it is possible to pass a
variable containing an association or an alist
+ when a sexp to be evaluated is required, it is possible to pass a
function instead
Merge pull request #54 from dudebout/variable-and-function-support

Enables using variables and functions as arguments
Removes `plist-get-sexp`

This function was not working as advertised.
Then `funcall` was evaluated too early and all the benefits of late evaluation for autoloads was lost.
Furthermore, this function was not really needed in the first place:

```
(use-package foo
  :config my-foo-function)
```

can easily be replaced by the following:

```
(use-package foo
  :config (my-foo-function))
```
Merge pull request #55 from dudebout/fixes-#54

Removes `plist-get-sexp`