Transfer XMR to address after swap

[da-kami]

• Remove buy-xmr command
• Move all the options of buy-xmr to the global scope
• Add obligatory parameter --xmr-address (try to use required if command is none of structopt to avoid having to specify it for other commands)

Note (Daniel): Once we do this we will have to make sure to wait the full 10 blocks on stagenet to be able to spend from the wallet created with generate_from_keys. At the moment this is set to 5 blocks which does not make much sense - it should either be 10 or 0 - if we spend it should be 10 if don't spend it can be 0, then the user has to make sure to wait the required 10 blocks to be able to spend.

[da-kami]

After giving this some more thought the conclusion is:

Unless we make providing the address an interactive step, the user has to specify the Monero address as part of the command. This means one cannot just start the application without providing zero parameters.

Given, that we have at least one parameter which is the address, I think it is actually more intuitive to keep the buy-xmr command and make the address a mandatory parameter on that, as in:

swap_cli buy-xmr --receive-address [xmr-address]

Besides minimalism I don't see an advantage in doing it like this:

swap_cli [xmr-address]

The assumption is, that the user will copy and paste the command provided on the homepage anyway (the homepage would provide a field where the xmr address is to be specified).

I will keep the buy-xmr command for now. If we get complaints about it I am happy to remove it :)