[Feature Request] import-name support snake_case file names

[dosentmatter]

https://github.com/Microsoft/tslint-microsoft-contrib/blob/d970a8a80134467cefd78f01ed49772506e9c21e/src/importNameRule.ts#L102

The current code allows periods, ., and hyphens, -, in file name imports and converts them to camelCase. Is it possible to allow underscores, _, as well?

I know underscores for file names isn't as common in javascript/typescript but I see it used sometimes. Currently, tslint would suggest import snake_case from 'snake_case'; and I think snake_case for variable naming is less common.

[JoshuaKGoldberg]

+1, this should be done. Seems like a nice and straightforward fix.