Keymaps not bound to data-grammar and conflict with other plugins

[billglover]

It would be good to limit the scope of the keymap to the graphviz data-grammar to prevent (or at least reduce) the possibility of a clash with other plugins.

There is currently a conflict when using the atom-tasks plugin. That plugin limits the scope of the Ctrl-D keymap to the todo data-grammar as below.

# For more detailed documentation see
# https://atom.io/docs/latest/advanced/keymaps
'.platform-darwin atom-workspace atom-text-editor:not(.mini)[data-grammar~="todo"]':
  'ctrl-d': 'tasks:complete'

In the graphviz-preview plugin there is no such limitation meaning the keymap is global.

# For more detailed documentation see
# https://atom.io/docs/latest/advanced/keymaps
'atom-workspace':
  'ctrl-d': 'graphviz-preview:toggle'

I would like to propose limiting the scope to the graphviz data-grammar. It may be possible to combine this enhancement with a fix for Issue #17.