Add strike through support

[hnOsmium0001]

Add strike through support.

• %X to enable strikethrough, and %R to reset as usual
• ~~text~~ if markdown is enabled
• x to insert strikethrough if shortcuts are enabled

The first item is currently possible with custom aliases, but the latter 2 can only be implemented with code modifications.

[hnOsmium0001]

Should be ready now. Accidentally committed the minimized html string into src/ the first time :P

I manually edited the minimized html string because the gulp version seem to be too old, and the solution over there doesn't seem to work. Not sure if the file in build/ is the same as the one gulp would've produced, but it does indeed to what I wanted.

[dogancelik]

Thank you for the PR and fixing the src file @hnOsmium0001. I need to update the npm packages, they are very old.