Add ViewMethods#cms_snippet_exists? method.

This method can be used when you need to render a snippet surrounded by some markup only if snippet exists:

    <% if cms_snippet_exists?(:sample_snippet) %>
      <div id="sample-snippet">
        <%= cms_snippet_content(:sample_snippet) %>
      </div>
    <% end %>

Also it is possible to render non-existing snippet without a block (default content):

    <%= cms_snippet_content(:not_existing_snippet) %> # => '' (empty string), no errors raised