Add gitignore template

[tbekolay]

Motivation and context:
Realized I needed to write a .gitignore for a new repo, so thought that it would be worth the extra few minutes to do it through Nengo Bones.

How has this been tested?
Added to the Bones .nengobones.yml, generated a new .gitignore.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• New feature (non-breaking change which adds functionality)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.

Still to do:

• The items in the above checklist
• Resolve the below discussion point

All-encompassing or minimal

Right now, I seeded this with Github's Python gitignore template, with some tools that we don't use take out and some tools that we do use added in. I haven't yet checked all downstream .gitignores for any additional things that would be generally useful. Before doing that (and writing tests, docs etc) I figured I would get some feedback on whether we want this style of .gitignore, which has probably more things in there than we need, or if we want a stripped down minimal .gitignore, which is what we have in most of our repos right now. The all-encompassing one should require fewer custom entries in .nengobones.yml at the cost of more lines in the generated .gitignore file.