feat: add spellcheck highlighting support

Introduce optional spellcheck highlighting in the input window using
the Enchant-2 library.

```
/spellcheck on
/spellcheck lang en_US
```

New theme color `input.misspelled`.

Fixes: https://github.com/profanity-im/profanity/issues/183
Signed-off-by: Michael Vetter <jubalh@iodoru.org>

src/config/preferences.h

@@ -161,6 +161,8 @@ typedef enum {
     PREF_STROPHE_SM_RESEND,
     PREF_VCARD_PHOTO_CMD,
     PREF_STATUSBAR_TABMODE,
+    PREF_SPELLCHECK_ENABLE,
+    PREF_SPELLCHECK_LANG,
 } preference_t;
 
 typedef struct prof_alias_t