Allows extra flexibility to the location of libtensorflow downloads.

[melmerp]

Specifically, modifies the install.js.

Adds two more options to control CDN_STORAGE for modifying the base
host:

1. TFJS_NODE_CDN_STORAGE env variable to help with any naming collisions
   with other packages that might be using CDN_STORAGE env variable.
2. TFJS_NODE_CDN_STORAGE in the .npmrc file

Adds two options to control the BASE_URI for use cases where the
url path doesn't match the default:

1. TJFS_NODE_BASE_URI env variable
2. TFJS_NODE_BASE_URI in the .npmrc

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This change is 

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[pyu10055]

Thank you for the contribution, looks good with couple minor comments.

[pyu10055]

This is bit duplicated, maybe better to add deprecation note on the CDN_STORAGE, since TFJS_NODE_CDN_STORAGE seems to be a better naming.

[melmerp]

Yes, I agree. It would be great to deprecate the use of CDN_STORAGE. Is there an "official" way to do this other than to leave a note that CDN_STORAGE will be deprecated at some future date? I will do this now.

[pyu10055]

You can put the option 2 and 3 at the top and move the CDN_STORAGE to the end and indicate it as a deprecated options.

[melmerp]

I've completed this. Thanks.

[melmerp]

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[pyu10055]

Thanks you!