Push mark before jumping to footnote

[slotThe]

Description

When inserting a (small) footnote, it is often very convenient to jump back to the main text with C-u C-SPC instead of using markdown-footnote-return. However, this can only be done if markdown-insert-footnote pushes to the mark ring before jumping, so do that.

Type of Change

• [n/a] Bug fix (non-breaking change which fixes an issue)
• [x] Improvement (non-breaking change which improves an existing feature)
• [n/a] New feature (non-breaking change which adds functionality)
• [n/a] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] I have read the CONTRIBUTING.md document.

• [n/a] I have updated the documentation in the README.md file if necessary.

• [n/a] I have added an entry to CHANGES.md.

  Not significant enough to warrant this, I think.

• [n/a] I have added tests to cover my changes.

  Not significant enough to warrant this, I think.

• [x] All new and existing tests passed (using make test).

[syohex]

org-footnote-new sets marker like this so I think this behavior looks good.