ENH: Add a gitignore template

[HaoZeke]

Closes #63. Maybe might be worthwhile adding windows and macos specific ignores to this?

[mattip]

I guess we could do this. I somewhat prefer to encourage people to use global .gitignore files for stuff on their systems: ide projects, editor backups and the such, and only have project-specific ignore files. But I can see why we might want to add something minimal here.

[HaoZeke]

I guess we could do this. I somewhat prefer to encourage people to use global .gitignore files for stuff on their systems: ide projects, editor backups and the such, and only have project-specific ignore files. But I can see why we might want to add something minimal here.

The Jetbrains stuff seems weird, but I thought it'd be easiest to take the Github defaults for python projects, rather than hand-rolling something for asv. I can see that OS specific stuff shouldn't be here though.