Allow more freedom for default highlights

[Felanbird]

In the development of Chatterino we've made certain decisions for users based on the consensus of developers, but some of these decisions removed user customization freedom. Specifically the ability to enable/disable: Show in mentions, Flash Taskbar, & Play Sounds for default highlights.

Should we allow for free customization of these default highlights? Since the user is already, or will become, familiar with the check-boxes based on custom highlights. (These new additions would all be disabled by default.)

From personal experience I know that the code additions for Play Sound & Flash Taskbar are basically copy paste, but I'm unsure about Show in Mentions.

Relevant: #2824 - #3370

[leibnaurd]

I think that, as a mod, having the sound notification play for first time chatters would be very helpful.