access Cmd elements as an array of strings

[stevengj]

Currently, a Cmd is a rather opaque object: there is no way to "look inside" it without accessing undocumented fields of the Cmd object. I ran into this recently when I wanted a function of a Cmd to do something different depending on the program being executed.

As a first step, this PR allows you to iterate and index a Cmd object as if it were an array of strings, e.g.:

julia> collect(`echo "foo bar"`)
2-element Array{String,1}:
 "echo"   
 "foo bar"

julia> `echo "foo bar"`[2]
"foo bar"

(This is just a thin wrapper around cmd.exec.)

[StefanKarpinski]

LGTM. I've considered doing this a number of times.

[tkelman]

Useful, but we are in feature freeze.

[StefanKarpinski]

Sure. We should probably branch after beta so that we can start merging features. The 1.0 release cycle will be very short already.

[tkelman]

Depends at what point waiting to merge features becomes more trouble than backporting all bugfixes. We have enough bugfixes continuing that I don't think we're there yet.

[StefanKarpinski]

We can also make bug fixes on the release branch and merged them forward to master.

[stevengj]

Now that there is a release-0.6 branch, can this be merged into master?

[stevengj]

merge?