Possibly remove `CMNode.stringContent`

[NightFlyer]

I think that stringContent is really just something that is internal to the parser and isn't really something that should be exposed as part of Maaku.

Note:

• cmark_node_get_string_content is defined in cmark-gfm-extension_api.h, not cmark-gfm.h as the other public methods are. The other methods in cmark-gfm-extension_api.h mostly deal with the parser. The only methods from there which are useful are the ones we need to get the syntax extension.
• the string content is never used in any of the rendering methods. This implies to me that it isn't used for any output, and so probably isn't useful for Maaku users.

At the very least, I think this all means that there is little reason to be able to set the value, and so Issue #45 shouldn't have to provide a setter function for the string content.