Expand `rustup show`

[brson]

Now it shows the active toolchain, the installed targets for the
active toolchain, and the installed toolchains. When there are
no additional toolchains or targets, the category headers
for them are not displayed.

[brson]

Here's what it looks like with all categories of info displayed, since the test cases aren't very readable:

brian@ip-10-234-25-118:~/dev/rustup.rs⟫ rustup show
installed toolchains
--------------------

stable-x86_64-unknown-linux-gnu
nightly-i686-unknown-linux-gnu
nightly-x86_64-unknown-linux-gnu (default)

installed targets for active toolchain
--------------------------------------

i686-unknown-linux-gnu
x86_64-unknown-linux-gnu

active toolchain
----------------

nightly-x86_64-unknown-linux-gnu (default)
rustc 1.10.0-nightly (8da2bcac5 2016-04-28)

And the headers are bold. But it might be better to make everything but the headers bold.

[brson]

Here's what it looks like with either the headers bold or the bodies.

[alexcrichton]

r=me, nice!

I kinda like the bold headers, but the bold toolchains looks a little too much