Bump version to 2.2.4

Since 2.2.3 we've renamed both the custom group and some "private" functions.
This really shouldn't change anything for anyone, but *maybe* it could, so we
should tag a version so people can revert more easily if they need to.

Fall back to define-key if keymap-set isn't bound

`keymap-set` was introduced in a recent version of Emacs. I switched to it in
2.2.2 to allow more complex keybindings, but forgot it's unavailable in older
versions! This checks whether `keymap-set` is available, uses it if it is, and
reverts to `define-key` if it's not.

Closes #57.

Allow binding engines to non-prefix key sequences

We'd like to be able to bind some engines to sequences of keys "nested" under
the common engine-mode prefix. For example, something like this ought to work:

```
(defengine wikipedia-spanish
    "https://es.wikipedia.org/w/index.php?search=%s"
    :keybinding "w s")
```

This makes that possible by replacing the use of `define-key` (which takes a key
sequence, which needs to start with a prefix key) with the recommended function
`keymap-set` (which takes any string that satisfies `key-valid-p`).

Closes #56.

Bump version to 2.2.1

Prevent browse-url-browser-function shadowing

Engines, by default, use the built-in `browse-url-browser-function` to decide
which browser to open for a given search. However, if that function has been
locally bound to another browser function, it'll use that new function instead!
That's not the intended behavior.

For example, suppose a user defines an engine to search Wikipedia using their
default browser, then tries to invoke it from within an `eww` buffer. They'd
*expect* their default browser to open, but instead, because `eww` locally
overwrites `browse-url-browser-function`, their search will be opened in `eww`!
Surprise!

This explicitly binds the `engine/browser-function` to
`browse-url-browser-function`. That ensures that the default browser will be set
at macro-expansion, so future rebindings of `browse-url-browser-function`
shouldn't affect engines.

Fixes #40.

Bump version to 2.1.2

Bump version to 2.1.1

v2.0.0

* Change the default key prefix from `C-c /` to to `C-x /`.
* Let the user customize that through engine/keybinding-prefix.

Release version 1.0.0