Add web-search plugin.

This plugin adds google, bing and yahoo commands to launch the default
web browser to do web search:

e.g.

    google oh-my-zsh
    bing what is zsh

plugins/web-search/web-search.plugin.zsh

@@ -0,0 +1,43 @@
+# web_search from terminal
+
+function web_search() {
+
+  # get the open command
+  local open_cmd
+  if [[ $(uname -s) == 'Darwin' ]]; then
+    open_cmd='open'
+  else
+    open_cmd='xdg-open'
+  fi
+
+  # check whether the search engine is supported
+  if [[ ! $1 =~ '(google|bing|yahoo)' ]];
+  then
+    echo "Search engine $1 not supported."
+    return 1
+  fi
+
+  local url="http://www.$1.com"
+
+  # no keyword provided, simply open the search engine homepage
+  if [[ $# -le 1 ]]; then
+    $open_cmd "$url"
+    return
+  fi
+
+  url="${url}/search?q="
+  shift   # shift out $1
+
+  while [[ $# -gt 0 ]]; do
+    url="${url}$1+"
+    shift
+  done
+
+  url="${url%?}" # remove the last '+'
+
+  $open_cmd "$url"
+}
+
+alias bing='web_search bing'
+alias google='web_search google'
+alias yahoo='web_search yahoo'