add parser plugin options for decorators

[connectdotz]

adding an optional parameter for parse function to allow users to override decorators plugin. For example:

parse(file, undefined, {plugins: {decorators: 'legacy'}}
parse(file, undefined, {plugins: {decorators: {decoratorsBeforeExport: false}}}

The same change has been made to Snapshot parse functions.

resolve #89

[connectdotz]

@firsttris do you want to look at this PR before the merge?

[connectdotz]

ok, I will merge first, a few other PR pending on this...