web-search: allow custom search engines

plugins/web-search/README.md

@@ -52,3 +52,27 @@ Also there are aliases for bang-searching DuckDuckGo:
 | `map`     | `!m`  |
 | `image`   | `!i`  |
 | `ducky`   | `!`   |
+
+### Custom search engines
+
+If you want to add other search contexts to the plugin, you can use the
+`$ZSH_WEB_SEARCH_ENGINES` variable. Set it before Oh My Zsh is sourced,
+with the following format:
+
+```zsh
+ZSH_WEB_SEARCH_ENGINES=(
+    <context> <URL>
+    <context> <URL>
+)
+```
+
+where `<context>` is the name of the search context, and `<URL>` a URL of
+the same type as the search contexts above. For example, to add `reddit`,
+you'd do:
+
+```zsh
+ZSH_WEB_SEARCH_ENGINES=(reddit "https://www.reddit.com/search/?q=")
+```
+
+These custom search engines will also be turned to aliases, so you can
+both do `web_search reddit <query>` or `reddit <query>`.