Load config before dispatching command

Update checker's behaviour is defined by the config file. So the configuration loading needs to be moved to the very beginning, before the update is checked.

I defined config.Config instance as a global variable. Thsi way one does not need to pass it as an argument to every function possible.
React to config's updates.check flag properly

Let's assume user user has attempted to run IPFS and got the error message suggesting her to change the updates.check value to 'ignore'.

We need to make sure the update check is not called when user attempts to do this with:

     ipfs config updates.check ignore
     ipfs update