Help adding a graph backend

[Kyle Andrews]

Dear Guix,

I am not very comfortable editing Guix source code. However, I would
very much like to add a new backend for `guix graph`. Right now guix
graph descriptions can only be exported into special purpose tools like
graphviz, D3, and cypher. I would like to add a fourth option which
would support loading the dependency data into general purpose network analysis 
software tools: an edgelist backend.

=> https://en.wikipedia.org/wiki/Edge_list

I think this code will do the job, but I don't know how to test it. So, I don't 
know for sure.

``` guix/graph.scm
(define (el-prologue name port)
  (display "from, to" port))

(define (el-epilogue port)
  (display "\n" port))

(define (el-node id label port)
  (display "" port))

(define (el-edge id1 id2 port)
  (format port "~a, ~a\n"))

(define %edgelist-backend
  (graph-backend "edgelist"
                 "Generate graph in CSV edge list format"
                 el-prologue el-epilogue
                 el-node el-edge))

(define %graph-backends
  (list %graphviz-backend
        %d3js-backend
        %cypher-backend
        %edgelist-backend)) ; <- the new proposed backend
```

Maybe it would be using some incantation involving ./pre-inst-env? I gave it a 
try following the manual:

```
cd ~/{{path/to/guix}}
guix shell -D guix
./bootstrap
./configure
```

This gave an error:

```
configure: error: chosen localstatedir '/usr/local/var' does not match that of 
the existing installation '/var'
Installing may corrupt /gnu/store!
Use './configure --localstatedir=/var'.
```

Since I am not that excited about corrupting my /gnu/store given that I don't 
know what I am doing, I didn't proceed further. 

I figured because I wanted to extend Guix with a new feature and that I
could piece together a story in my head about how the code should look, my 
query should be sent to this list rather than to help-guix.

Thanks in advance for any helpful suggestions towards getting this backend 
added to Guix!

Cheers,
Kyle