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.

cypress/integration/rendering/flowchart.spec.js

@@ -696,4 +696,22 @@ describe('Flowchart', () => {
       {  }
     );
   });
+
+  it('35: should honor minimum edge length as specified by the user', () => {
+    imgSnapshotTest(
+      `graph TD
+      L1 --- L2
+      L2 --- C
+      M1 ---> C
+      R1 .-> R2
+      R2 <.-> C
+      C -->|Label 1| E1
+      C -- Label 2 ---> E2
+      C ----> E3
+      C -----> E4
+      C ======> E5
+      `,
+      {  }
+    );
+  });
 });