Move SpewState methods to ConfigState.

Rather than stuffing a ConfigState instance into a separate SpewState,
just add the functionality directly to the ConfigState.  This provides
simpler syntax for the consumer.

One side effect of this change is that, unlike a zero value SpewState, a
zero value ConfigState doesn't provide default values which means the
Indent field is set to provide no indentation.  The consumer is now
expected to set the indent to their desired value when declaring an
instance of ConfigState.

Alternatively, the consumer can call a new function, NewDefaultConfig,
which returns a ConfigState with default values, including a default
indentation of a single space.

For example, to change the indent to a tab, the previous syntax was:

ss := new(spew.SpewState) // or var ss spew.SpewState
scs := ss.Config()
scs.Indent = "\t"
scs.Dump(whatever)

The new syntax is simply:

scs := spew.ConfigState{Indent: "\t"}
scs.Dump(whatever)