Specifically look in the home dir for configuration

Some workspaces, such as windows, often have the home directory not in the
hierarchy of where projects live, which means that configuration may not work by
default. The home directory, however, is already singled out for `cargo login`
and it's pretty semantically special, so this commit ensures that we always look
at the home directory for configuration if it exists.

Closes #933
auto merge of #946 : alexcrichton/cargo/issue-933, r=brson

Some workspaces, such as windows, often have the home directory not in the
hierarchy of where projects live, which means that configuration may not work by
default. The home directory, however, is already singled out for `cargo login`
and it's pretty semantically special, so this commit ensures that we always look
at the home directory for configuration if it exists.

Closes #933