Support for null safety

[mzdm]

• 1. Publish 1.0.0 version of the current package (v1.0.0 release #9)
• 2. Migrate code to null safety in a new branch
• 3. Publish null safety version as pre-release

Known dependencies currently blocking publishing:

• Migration to null safety dart-lang/http#501
• path_provider Migrate path_provider to null safety. flutter/plugins#3460

// Edit: packages have already migrated

Dev dependencies (does not block publishing):

• mustache
• console

[mzdm]

✔ null safety published & nnbd branch available here