Format lesson sequence number with leading 0's for sortability

[lyricnz]

That way the files sort naturally, and can be added to playlists etc in the right order. Since we know how many lessons are present before they are downloaded, the width of the format-string can be dynamically set to the number of digits in the biggest number (usually will be 2).

[lyricnz]

Here's a commit from my fork which does this https://github.com/RahulShaw/LinuxAcademy-DL/commit/eac5c61fae7e9ca4fc1382cd9a3285be9f367c56 No PR since I've already merged the platform/path fix mentioned previously