fromWei/toWei equivalents

[ricmoo]

We need to be able to convert to/from wei/ether. I will likely copy most of the functionality from the existing ethers.io formatEther.

• formatEther(weiAsBnOrHex, options) => decimalString
• parseEther(stringAsDecimalOrNumber) => BN

Options:

• round: 'up' | 'down' | 'round' | 'none' (default: none)
• decimals: Number (default: 18)
• commify: true | false (default: false)

[ricmoo]

Done, with some slight adjustments to the options available.