Sample remarkable plugin does not match content beginning with a mention

[jpribyl]

https://github.com/Rosey/markdown-draft-js/blob/0a8059cfd03f3678c5dd6c311a50acad16c28756/test/markdown-to-draft.spec.js#L372

0 is falsey in JS, so this will not match a mention at the start of an editor.

A better idea here would be to use something like:

        if (!state.src || typeof state.pos !== 'number') {

A test case could then be added:

  it('can handle editor beginning with block entity data', function () {

Alternatively, it could just be a second check in the existing test:

  it('can handle block entity data', function () {

[jpribyl]

If desired, I can submit a PR with this change