Add pandoc support

Here I try to fix an another warning from tests.
```
cannot load such file -- pandoc-ruby: skipping markdown tests with
Tilt::PandocTemplate
```

`pandoc-ruby` is not defined in Gemfile, so I add it there.
This gem has a prerequisite dependency `pandoc` application that can be
installed via homebrew. That's why I update travis.yml to make it
available during a test run.

The final step to make it work is to exclude `outvar` from the option's
list provided to the template. When `pandoc` application is called
with unsupported parameters it stops the execution and returns an
error.

.travis.yml

@@ -17,7 +17,7 @@ addons:
   - ubuntu-toolchain-r-test
   packages:
   - g++-4.8
-
+  - pandoc
 rvm:
   - 2.2.10
   - 2.3.8

Gemfile

@@ -60,6 +60,7 @@ if RUBY_ENGINE == "ruby"
   gem 'reel-rack'
   gem 'celluloid', '~> 0.16.0'
   gem 'commonmarker', '~> 0.20.0'
+  gem 'pandoc-ruby', '~> 2.0.2'
   gem 'simplecov', require: false
 end
 

README.md

@@ -750,6 +750,8 @@ template, you almost always want to pass locals to it.
         <a href="https://github.com/ged/bluecloth" title="BlueCloth">BlueCloth</a>,
         <a href="https://kramdown.gettalong.org/" title="kramdown">kramdown</a>,
         <a href="https://github.com/bhollis/maruku" title="maruku">maruku</a>
+        <a href="https://github.com/gjtorikian/commonmarker" title="commonmarker">commonmarker</a>
+        <a href="https://github.com/alphabetum/pandoc-ruby" title="pandoc">pandoc</a>
     </td>
   </tr>
   <tr>

lib/sinatra/base.rb

@@ -722,6 +722,7 @@ module Sinatra
     end
 
     def markdown(template, options = {}, locals = {})
+      options[:default_outvar] = false
       render :markdown, template, options, locals
     end
 
@@ -818,10 +819,11 @@ module Sinatra
       content_type    = options.delete(:content_type)   || content_type
       layout_engine   = options.delete(:layout_engine)  || engine
       scope           = options.delete(:scope)          || self
+      default_outvar  = options.delete(:default_outvar) != false
       options.delete(:layout)
 
       # set some defaults
-      options[:outvar]           ||= '@_out_buf'
+      options[:outvar] ||= '@_out_buf' if default_outvar
       options[:default_encoding] ||= settings.default_encoding
 
       # compile and render template