Update JetBrains.gitignore

To clarify when to ignore/include *.imls as per https://intellij-support.jetbrains.com/entries/23393067

also added optionals from the same
Updated following feedback

removed the iml filter - this is my current best understanding of the docs, but I'm still unsure
change iml

guess that root iml needs to be ignored and others need to be shared
Merge pull request #1137 from samskiter/master

Update JetBrains.gitignore