Storage write streams should be able to use a pre-existing uri

[bhstahl]

Allow users to pass through an already existing uri when creating a write stream.

// create resumable url
file.createResumableUpload((err, uri) =>  {
    // hang on to uri somehow
});

// create a new stream with the same uri
const config = { uri };
bucket.file('zebra.jpg').createWriteStream(config);

Closes #1321

[stephenplusplus]

Thank you! Could you add a line in the jsdocs for:

• Bucket#upload
• File#createWriteStream

Something like:

 * @param {string} options.uri - The URI for an already-created resumable
 *     upload. See {module:storage/file#createResumableUpload}.

[bhstahl]

Ah yes, good call @stephenplusplus. Updated!

[coveralls]

Coverage remained the same at 100.0% when pulling 006754c on bhstahl:storage-uri into 17e510b on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 006754c on bhstahl:storage-uri into 17e510b on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 006754c on bhstahl:storage-uri into 17e510b on GoogleCloudPlatform:master.

[stephenplusplus]

Coveralls is going crazy again. Sorry I missed this the first time, could you add a test to check that the uri option is passed through to gcs-resumable-upload?

[stephenplusplus]

Also, just released [email protected] with URI support -- can you upgrade the package.json to use this new one?

[bhstahl]

👍 on it @stephenplusplus

[coveralls]

Coverage remained the same at 100.0% when pulling 9a6afeb on bhstahl:storage-uri into 17e510b on GoogleCloudPlatform:master.

[stephenplusplus]

Merged in via 3d6fa8a. Thanks @bhstahl!