Add contract details command

Created by: RNabel

Adds an endpoint for contract details, and a siac command to view details of a contract. Adresses #1620 (closed).

The endpoint is at: GET /renter/contract/*contract-id. It returns a json-serialized modules.RenterContract object.

The siac command is: siac renter contracts view <contract-id>

The output is currently:

OVERVIEW
========
End height:             107385
Host:                   x.x.x.x:9982
ID:                     xxxxxxxxxx7c56f90b7cc0a8ef31c27792bd2bb8cc4d5c468ae5426bec5

FILE CONTRACT
=============
File Size:              0 B

SPENDING
========
Renter funds:           2.329 SC
Upload Spending:        7.565 mS

This is a work in progress - let me know what else should be included. See here which information can be printed.