Add postcss-single-charset plugin
Add postcss-single-charset plugin

[postcss-single-charset][1] is for popping first `@charset` rule to the top of CSS file. This plugin is for fixing wrong `@charset` rule position after concatenate CSS files.

[1]: https://github.com/hail2u/postcss-single-charset
KivyGogh/postcss,justim/postcss,bendtherules/postcss,mxstbr/postcss,johnotander/postcss,chengky/postcss,MohammadYounes/postcss,webdev1001/postcss,LucasFrecia/postcss,ajoslin/postcss,jonathantneal/postcss,notacouch/postcss,mahtd/postcss,Shugar/postcss,johnie/postcss,sandralundgren/postcss,CapeSepias/postcss,longdog/postcss,bezoerb/postcss,mshahidkhan/postcss,Paddy-Hamilton/postcss,pascalduez/postcss,gitkiselev/postcss,zhuojiteam/postcss,pazams/postcss,marek-saji/postcss,nicksheffield/postcss,greyhwndz/postcss,phillipalexander/postcss,whitneyit/postcss,himynameisdave/postcss,smagic39/postcss,admdh/postcss,postcss/postcss,alanev/postcss,webdev1001/postcss,qiucanghuai/postcss,omeripek/postcss,alexmchardy/postcss,MadLittleMods/postcss,mdegoo/postcss,dehuszar/postcss,eprincev-egor/postcss,asan/postcss,Semigradsky/postcss,andrepolischuk/postcss,maximkoretskiy/postcss,c2ye/postcss,marcustisater/postcss,dpiatek/postcss,jedmao/postcss,wjb12/postcss,postcss/postcss,kk9599/postcss,XOP/postcss,cnbin/postcss,dpostigo/postcss,cbas/postcss,harrykiselev/postcss,dmsanchez86/postcss,geminiyellow/postcss,ashelley/postcss,f/postcss,rtsao/postcss,OEvgeny/postcss