text: Title: Support sqlite database urls relative to project path. 
  Body: When developing it makes sense to have the database file in the local repository directory, to have a `dev.env` which is valid for all developers it needs to be relative..
  
  I have been doing hacks in settings.py to get around it for a few years now but I would like a discussion about how to maybe integrate such a feature into django-environ.
  
  This basically just assumes that if the the file is at the root of the system it's instead relative but I'm looking for a way to get rid of these in every project..
  ```py
  for db in DATABASES:
      if DATABASES[db]['ENGINE'] == 'django.db.backends.sqlite3':
          if os.path.split(DATABASES[db]['NAME'])[0] == "/":
              DATABASES[db]['NAME'] = root(DATABASES[db]['NAME'][1:])
  ```
  
  I am thinking that this is kind of sane because no one proably stores the database in the root of the file system but making it optional by having a `sqlite_root` argument to the db-url function maybe works? 
  
  ```py
  root = environ.Path(__file__) - 2
  ...
  DATABASES = {
      'default': env.db(sqlite_root=root()),
  }
  ```
  
  I think that both relative AND absolute paths should work.
  
  One option which I think might work safely for triggering a relative path instead of absolute is to do it when host is `.` or some other reserved name which isn't really a hostname. It should error out if `sqlite_root` was not provided to the url parse function.
  ```py
  DATABASE_URL=sqlite://./db.sqlite3
   ```
  
  Please share your opinions on this..