Add option for post-processing command/script

[peskdale]

I wanted a way to send a notification when a new video landed. The easiest way I could think to do it was to just have a user-defined command be run after completion.

This reuses the same string replacements of filePathFormatting, so I have broken that out into a separate function.

Disclaimer: other than odd tweaks to existing code, I have never done any proper work with typescript before. Feel free to make changes to the code to suit standards and/or personal taste.

[Inrixia]

Looks good, Ill probably add a proper example in the wiki on how to use it before the next release.

[Inrixia]

Ill also need to add some error handling as if the command fails the download shouldn't fail. re issue #79
And swapout the exec to a use util.promisify as seen below (basically the same as what you did):

import { exec as execOriginal } from "child_process";
import { promisify } from "util";
const exec = promisify(execOriginal);

Otherwise thanks for the added functionality :)