Load a desk without launching a new shell instance

It would be nice if there was a `desk load` or some such subcommand which would load a desk's script without leaving the current shell instance. This would be useful for allowing one to share some common desk setup among other desks. For instance, say I want to have some script setup some environment variables that will be common to other desks as well. Currently, as far as I can tell, the only way to do this is with something like:

``` sh
# load the common code
. $(dirname $0)/common.sh

# start setting up the subdesk
```

when it would be much nicer just to be able to do something like:

``` sh
# load the common code
desk load common

# start setting up the subdesk
```

Start by reading the existing desk subcommand handling and how desk scripts are currently loaded. The proposed `desk load common` command should load a desk's script into the current shell without launching a new shell instance, allowing shared setup such as environment variables; verify this with the common.sh example.

Written by the indexing model from the issue text.