Add support for custom MIME TYPES in karma.config.js

[comdiv]

For example - default mime.js maps *.ts is video\mp2t.
While we don't have to fix mime.js itself we should be able to provide ability to set mime explicitly in config.

Advice - use format close to mime.js:

{
    mime : {
            "text/x-typescript":["ts","tsx"]
     },
}

Each property maps to mimetype and array value to extensions.

Have implement it myself. Pull request is coming