add markdown table center css class
add markdown table center css class

jay-hodgson/SynapseWebClient,jay-hodgson/SynapseWebClient,Sage-Bionetworks/SynapseWebClient,Sage-Bionetworks/SynapseWebClient,Sage-Bionetworks/SynapseWebClient,jay-hodgson/SynapseWebClient,Sage-Bionetworks/SynapseWebClient,jay-hodgson/SynapseWebClient