Add --delete option to push command

[AMeng]

The sync command is used to sync remote monitor state with local monitor state. This means creating, updating, and deleting monitors as needed.

Fixes #8 and, assuming all monitors are tracked in code, #7 as well.

[thesamet]

Thanks for putting the PR together. I prefer this behavior to be implemented as a flag to push. Can it be dogpush --delete_untracked ?

[AMeng]

Delete output:

$ dogpush push --delete
Pushing 3 new monitors.
Updating 1 modified monitors.
Deleting 2 untracked monitors.

Updated help output:

$ dogpush --help
usage: dogpush [-h] [-c CONFIG] {init,push,diff,mute} ...

positional arguments:
  {init,push,diff,mute}
                        sub-command help
    init                Init new alerts file
    push                Push monitors to DataDog.
    diff                Show diff between local monitors and DataDog
    mute                Mute alerts based on their `mute_when` key

optional arguments:
  -h, --help            show this help message and exit
  -c CONFIG, --config CONFIG
                        configuration file to load (default: ./config.yaml)

Updated push help output:

$ dogpush push --help
usage: dogpush push [-h] [-d]

optional arguments:
  -h, --help    show this help message and exit
  -d, --delete  Delete untracked monitors.

[AMeng]

@thesamet changed to a flag.

[thesamet]

delete_untracked

[thesamet]

here too

[thesamet]

@nandrienko Reviewed this. Please merge when convenient.