Andrew Connell walks through the Office 365 Patterns & Practices Scenario provisioning SharePoint wiki pages and manipulating the contents. This code can be found on GitHub here https://github.com/OfficeDev/PnP/tree/master/Samples/Core.ModifyPages
The sample uses the SitePages library which is default available in non-publishing sites, but the same sample can be easily adapted to create pages in a custom wiki page library.
In the last step, the empty wiki page is filled with HTML content. In the sample you can specify your HTML content and then click Run scenario 1 to create the wiki page and add the HTML