Add feature for adding keys by environment variables

[vaithak]

Fixes #104 .

[vaithak]

@diwakergupta Please review my PR.

[diwakergupta]

Unfortunately this breaks the current build system for dbxcli -- see how credentials are supplied at build-time via build.sh. Can you move updating all these to init instead?

[vaithak]

@diwakergupta I have made the requested changes, now the keys can be accessed through env variables without build failure in official release. Please review my PR.

[clarkewd]

Thanks for adding this feature! I've looked over the changes and also the code in root.go, and read the comments here and on #104

I'm still a little confused though. Is it possible to continue to use the default app keys (i.e. not create my own dropbox app) and then use either the web authentication response or the data from the ~/.config/dbxcli/auth.json file set as an environmental variable instead of using the config file?

I've tried it with both DROPBOX_PERSONAL_APP_KEY and DROPBOX_PERSONAL_APP_SECRET but this doesn't seem like the right place for it...