add a MigrateList class for CSV source

Very pleased to find someone had already implemented this, I think it's a really good addition to the standard set.

I had to make some updates to get this working as specified for CSV with header rows and using header rows as keys.

Some observations

1. +++ b/plugins/sources/csv.inc
   @@ -6,6 +6,173 @@
   +   * @var integer
   

   Needs a parameter comment

2. +++ b/plugins/sources/csv.inc
   @@ -6,6 +6,173 @@
   +    // fgetcsv specific options
   ...
   +   * @return string
   

   Missing period. Needs to be capitalised

3. +++ b/plugins/sources/csv.inc
   @@ -6,6 +6,173 @@
   +   * @return string
   ...
   +   */
   

   Needs a return comment

4. +++ b/plugins/sources/csv.inc
   @@ -6,6 +6,173 @@
   +   */
   

   Needs a return type and comment

5. +++ b/plugins/sources/csv.inc
   @@ -6,6 +6,173 @@
   +
   

   Missing doc block

Further observations

1. +++ b/plugins/sources/csv.inc
   @@ -34,6 +34,13 @@
   +   */
   

   Missing variable comment.

2. +++ b/plugins/sources/csv.inc
   @@ -141,12 +156,16 @@
   +    // Key the array with the headers
   

   Missing period at end of comment.

I haven't capitalised "// fgetcsv specific options." as fgetcsv is a function name.
Sorted the rest of these code style issues.

(Also this was copied from the matching JSON function which is already in the module.)

+++ b/plugins/sources/csv.inc
@@ -6,6 +6,181 @@
+    parent::__construct();

Constructers should only really be used for defining class variables, could the functionality below 66 be moved into a new method? apart from that all good

Given all of the other migrate source plugins use the constructor in the same way, I'd suggest that's a more general code style request that you could make in a separate issue?

OK if your happy with that...

Done