Refactor option support

Define the extension options separate from the general options.

Test for the presence of an option before trying to evaluate it. This
protects against options being unset after they were first initialized.

autoload/preview.vim

@@ -70,16 +70,19 @@ require 'shellwords'
 class Preview
   include Singleton
 
+  EXT_OPTIONS = {
+    :markdown => "g:PreviewMarkdownExt",
+    :textile  => "g:PreviewTextileExt",
+    :rdoc     => "g:PreviewRdocExt",
+    :ronn     => "g:PreviewRonnExt",
+    :html     => "g:PreviewHtmlExt",
+    :rst      => "g:PreviewRstExt"
+  }
+
   OPTIONS = {
     :browsers     => "g:PreviewBrowsers",
-    :css_path     => "g:PreviewCSSPath",
-    :markdown_ext => "g:PreviewMarkdownExt",
-    :textile_ext  => "g:PreviewTextileExt",
-    :rdoc_ext     => "g:PreviewRdocExt",
-    :ronn_ext     => "g:PreviewRonnExt",
-    :html_ext     => "g:PreviewHtmlExt",
-    :rst_ext      => "g:PreviewRstExt"
-  }
+    :css_path     => "g:PreviewCSSPath"
+  }.merge!(EXT_OPTIONS)
 
   DEPENDECIES = {
     # :format => {:gem => 'name of gem'  , :require => 'file to require'}
@@ -92,10 +95,9 @@ class Preview
 
   def show
     update_fnames
-    ext_opts = OPTIONS.keys.find_all{|k| k.to_s =~ /_ext$/}
-    ext_opts.each do |opt|
-      if exts_match?(option(opt).split(','))
-        send "show_" + opt.to_s[/^(.*)_ext$/, 1]
+    EXT_OPTIONS.keys.each do |opt|
+      if has_option(opt) and exts_match?(option(opt).split(','))
+        send "show_#{opt}"
         return
       end
     end
@@ -254,6 +256,11 @@ class Preview
     VIM.evaluate(OPTIONS[name])
   end
 
+  def has_option(name)
+    raise "Unknown option #{name.inspect}" unless OPTIONS.keys.include?(name)
+    VIM.evaluate("exists('#{OPTIONS[name]}')") != 0
+  end
+
   def load_dependencies(format)
     require DEPENDECIES[format][:require]
     true