More explicit buttons & global config

[rpeyron]

Hello,

A very small PR with:

• More explicit buttons (some people using the plugin lost data because of misunderstanding):
  • "Cancel" instead of "Close" ; it will also check if the contents have been modified and ask for confirmation if modified and not saved
  • "Save" because it is sometimes useful to save without closing (for instance in the middle of a big sequence diagram)
  • "Save and quit" will save and quit :)
• The addition of another source of global config (window.defaultMermaidConfig that can be set in PreConfig.js), useful when bundled in a specific drawio distribution to include specific configuration that will apply even if there is some user local configuration

Hope his will suits you

Best regards

[rpeyron]

Also now fix #127