Allow align attributes in Markdown READMEs

Github, pypi.org, npmjs.com... allow the usage of align attribute for alignment. The sanitization filter defined in the README of this repository points to https://github.com/jch/html-pipeline/blob/master/lib/html/pipeline/sanitization_filter.rb, which also has implemented this.

So, shouldn't this be allowed. Currently, it seems that there is no way to center-align elements in Gitlab's READMEs.