Lazy-load viz.js

This commit moves the import of viz.js into a `require.ensure` block,
that is only executed when a graphviz diagram is actually present
in a note. Webpack automatically splits the library into a separate
chunk and loads that on demand.

To ensure that graphviz code-blocks are not treated as normal
code-blocks while the chunk is loading, a corresponding check is added
to `finishView`.

The library is also removed from the Webpack config file, as it only
is used at one place in extra.js, which is handled by Webpack
without any extra config.

Signed-off-by: David Mehren <git@herrmehren.de>

webpack.common.js

@@ -253,7 +253,6 @@ module.exports = {
       'expose-loader?exposes=LZString!lz-string',
       'flowchart.js',
       'js-sequence-diagrams',
-      'expose-loader?exposes=Viz!viz.js',
       'expose-loader?exposes=io!socket.io-client',
       'expose-loader?exposes=RevealMarkdown!reveal-markdown',
       path.join(__dirname, 'public/js/index.js')
@@ -284,7 +283,6 @@ module.exports = {
       'expose-loader?exposes=emojify!emojify.js',
       'flowchart.js',
       'js-sequence-diagrams',
-      'expose-loader?exposes=Viz!viz.js',
       'expose-loader?exposes=RevealMarkdown!reveal-markdown',
       path.join(__dirname, 'public/js/pretty.js')
     ],
@@ -318,7 +316,6 @@ module.exports = {
       'expose-loader?exposes=emojify!emojify.js',
       'flowchart.js',
       'js-sequence-diagrams',
-      'expose-loader?exposes=Viz!viz.js',
       'expose-loader?exposes=Reveal!reveal.js',
       'expose-loader?exposes=RevealMarkdown!reveal-markdown',
       path.join(__dirname, 'public/js/slide.js')
@@ -342,7 +339,6 @@ module.exports = {
   },
 
   externals: {
-    'viz.js': 'Viz',
     'socket.io-client': 'io',
     'jquery': '$',
     'moment': 'moment',