IPFS_API environment variable

[kallisti5]

Instead of passing --api="/ip4/192.168.1.10/tcp/45001" to ipfs every execution, it would be nice if it accepted an environment variable for this setting.

IPFS_API or something similar. This would likely be handy when running ipfs in docker containers connecting to a central node as well.

IPFS_PATH is already offered... so seems like a reasonable request.

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[Stebalien]

Note: you can also create a ~/.ipfs/api file with the correct API path. But I agree that an environment variable would be nice.

[zaibon]

I did a quick search to see where we could hook this environment variable. It seem we could modify https://github.com/ipfs/go-ipfs/blob/48b2f1273af310934d0e2334ce5460712e706200/cmd/ipfs/main.go#L193-L199
to also check IPFS_API if the --api flag is not given.

I guess we want the flag to have precedence over the environment variable?
If this plan sounds good I can work on a PR for it.