Add sync to dev flag as a flag to awsmobile pull

[calebfroese]

Issue #, if available:

142

Description of changes:

The $ awsmobile pull command now accepts -d/--sync-dev set the syncToDevFlag, which is responsible for prompting the user on how to synchronise local files with those in AWS.

Usage

The default value remains 0, "prompt for confirmation" $ awsmobile pull

Manually specify the syncToDevFlag to not sync to backend $ awsmobile pull --sync-dev=-1 or $ awsmobile pull -d=-1

etc

Help

The output from $ awsmobile pull --help is now:

  Usage: awsmobile-pull [options]

  pull the latest details of the awsmobile project from the cloud

  Options:

    -d, --sync-dev <value>  synchronize contents in #current-backend-info/ with the latest in the aws cloud (default: 0)
    -h, --help              output usage information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #143 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #143   +/-   ##
=======================================
  Coverage   80.85%   80.85%           
=======================================
  Files          85       85           
  Lines        5700     5700           
  Branches      914      914           
=======================================
  Hits         4609     4609           
  Misses        932      932           
  Partials      159      159

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