Add support for changing the length of edges

Resolves #637, resolves #1495.

The syntax for edges is extended to allow for extra
dashes/equals signs/dots (depending on the edge type). Each added
character increases the length of the edge by one rank level, as
follows:

Length            |    1   |    2    |     3    |
------------------|:------:|:-------:|:--------:|
Normal            |  `---` |  `----` |  `-----` |
Normal with arrow |  `-->` |  `--->` |  `---->` |
Thick             |  `===` |  `====` |  `=====` |
Thick with arrow  |  `==>` |  `===>` |  `====>` |
Dotted            |  `-.-` |  `-..-` |  `-...-` |
Dotted with arrow | `-.->` | `-..->` | `-...->` |

This features leverages the `minlen` property exposed by Dagre which was
not previously available when using Mermaid.

src/diagrams/flowchart/flowRenderer.js

@@ -244,6 +244,7 @@ export const addEdges = function(edges, g) {
 
     edgeData.id = linkId;
     edgeData.class = linkNameStart + ' ' + linkNameEnd;
+    edgeData.minlen = edge.length || 1;
 
     // Add the edge to the graph
     g.setEdge(edge.start, edge.end, edgeData, cnt);