onLanguage:mermaid doesn't seem to be working

[depascalis]

Describe the bug The extension isn't activated on mermaid language detection.

To Reproduce Steps to reproduce the behavior:

1. Create a new untitled file in VSCode, do not save it
2. Write some mermaid code

graph TD
  X[Hello]

3. Make sure that the correct language is detected
4. Open the Command Pallet and search for "mermaid" -> nothing appears
5. Look at the top right of the document window -> no preview icon is visible

Expected behavior Previewing should work on untitled files.

Desktop (please complete the following information):

• OS: Windows 10
• VSCode 1.7
• Extension version 0.15.0

Additional context bpruitt-goddard.mermaid-markdown-syntax-highlighting 1.4.0 is also enabled