Issue templates for Cirq repo would benefit from updates

[mhucka]

Description of the issue

The issue templates currently don't take advantage of GitHub features. Two in particular are:

• Formatting: the issue templates use Markdown bold-face ** format directly instead of using heading formats for headings. Using ## for headings would make the templates a little easier to read, IMHO
• GitHub supports issue forms and we should use them to ask the questions that are currently asked as text in the issue templates.

[mhucka]

I recently did the first item (improving the Markdown). Still need to do the second.