Allow for custom directory to be passed to `dapp install`

[asymmetric]

dapp install --dir foo would override lib/.

[rainbreak]

These can all be overridden:

export DAPPTOOLS=${DAPPTOOLS-~/.dapp/dapptools}
export DAPP_SRC=${DAPP_SRC-src}
export DAPP_LIB=${DAPP_LIB-lib}
export DAPP_OUT=${DAPP_OUT-out}

[asymmetric]

dapp install doesnt use it though.