LibGemini: Implement rendering text/gemini documents to HTML

This also sets Content-Type to whatever 'meta' contains on success, to
allow the browser to pick up what the document contains.

Libraries/LibGemini/CMakeLists.txt

@@ -1,8 +1,10 @@
 set(SOURCES
+    Document.cpp
     GeminiJob.cpp
     GeminiRequest.cpp
     GeminiResponse.cpp
     Job.cpp
+    Line.cpp
 )
 
 serenity_lib(LibGemini gemini)