sage run should be graceful if you omit the suffix

[zepumph]

I'd like to improve on sage run a bit so that in simple, common cases, you don't need to add the file extension. For example, in #433, I noticed that you run a script like sage run perennial/js/scripts/push-all.js. As part of that issue, I wanted to convert it to typescript, but that would be an API breakage for all users. Instead, I would prefer to tell people to just use sage run perennial/js/scripts/push-all, and it could have the following algorithm order:

1. Try to find the path directly
2. Try to find the path with ".ts" added on
3. Try to find the path with ".js" added on
4. Fail out.

Currently, it fails out if you don't have a file suffix on it. I have found that awkward, and a bad parallel for grunt replacement. I really can't think of a spot where this would be confusing or result in a false positive or an incorrect behavior.

Tagging @samreid, but unless I hear objections, I will proceed with the graceful implementation.