Add support for match-all custom selectors

[jefflembeck]

Examples:

customselectors: {
  "*": [".icon-$1:before", ".icon-$1-what", ".hey-$1"]
},
prefix: ".icon-"

Should give the file bear.svg the css

.icon-bear:before,
.icon-bear-what,
.hey-bear,
.icon-bear {
 // CSS THINGS
}

And if there are files bear.svg and cat.svg, the css should be like:

.icon-bear:before,
.icon-bear-what,
.hey-bear,
.icon-bear {
 // CSS THINGS
}

.icon-cat:before,
.icon-cat-what,
.hey-cat,
.icon-cat {
 // CSS THINGS
}

[jefflembeck]

This was originally started over here - filamentgroup/grunticon#84