Add dapp key command to create wallets

**Issue by [nanexcool](https://github.com/nanexcool)**
_Thursday Mar 15, 2018 at 23:09 GMT_
_Originally opened as https://github.com/dapphub/dapp/pull/86_

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`dapp key --help`

`dapp key --keystore path` (will prompt for password)

`dapp key --keystore path --password file`

default keystore `~/.ethereum/keystore`

can be used with `ETH_KEYSTORE` and `ETH_PASSWORD` for `seth` compatibility

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_**[nanexcool](https://github.com/nanexcool)** included the following code: https://github.com/dapphub/dapp/pull/86/commits_

Start by reviewing the dapp/pull/86 commits and the behavior described by `dapp key --help`. Verify the `--keystore` and `--password` options, the default `~/.ethereum/keystore` path, and compatibility with `ETH_KEYSTORE` and `ETH_PASSWORD`; done means the command creates wallets and prompts for a password when required.

Written by the indexing model from the issue text.