Allow the user to provide stable node-ids

[vringar]

When commit the diagrams to git and making sure they are up-to-date by running diagrams in CI, it is annoying that each new run generates new _ids for the nodes.
See:

diagrams/diagrams/__init__.py

Line 292 in a03b305

self._id = self._rand_id()

I would be nice, if a user could specify a nodes _ids to ensure that the output only changes on a change to diagrams or the input file.

Closed by #422

[vringar]

Current output of my git diff:

-<!-- 1cd0443688924b0582222605b360a543 -->
+<!-- 9aa846bb6f224d28a3d71379889d9ea6 -->
<g id="node1" class="node">
-<title>1cd0443688924b0582222605b360a543</title>
+<title>9aa846bb6f224d28a3d71379889d9ea6</title>

for every node in my graph.