RST reader: implement csv-table directive.

Most attributes are supported, including `:file:` and `:url:`.
A (probably insufficient) test case has been added.

Closes #3533.

test/command/3533-rst-csv-tables.csv

@@ -0,0 +1,4 @@
+"Albatross", 2.99, "On a stick!"
+"Crunchy Frog", 1.49, "If we took the bones out, it wouldn't be
+crunchy, now would it?"
+

test/command/3533-rst-csv-tables.md

@@ -0,0 +1,55 @@
+```
+% pandoc -f rst -t native
+.. csv-table:: Test
+   :widths: 10, 5, 10
+   :header: Flavor,Price,Slogan
+   :file: command/3533-rst-csv-tables.csv
+^D
+[Table [Str "Test"] [AlignDefault,AlignDefault,AlignDefault] [0.4,0.2,0.4]
+ [[Plain [Str "Flavor"]]
+ ,[Plain [Str "Price"]]
+ ,[Plain [Str "Slogan"]]]
+ [[[Plain [Str "Albatross"]]
+  ,[Plain [Str "2.99"]]
+  ,[Plain [Str "On",Space,Str "a",Space,Str "stick!"]]]
+ ,[[Plain [Str "Crunchy",Space,Str "Frog"]]
+  ,[Plain [Str "1.49"]]
+  ,[Plain [Str "If",Space,Str "we",Space,Str "took",Space,Str "the",Space,Str "bones",Space,Str "out,",Space,Str "it",Space,Str "wouldn't",Space,Str "be",SoftBreak,Str "crunchy,",Space,Str "now",Space,Str "would",Space,Str "it?"]]]]]
+```
+
+```
+% pandoc -f rst -t native
+.. csv-table:: Test
+   :header-rows: 1
+   :quote: '
+   :delim: space
+
+   '' 'a' 'b'
+   'cat''s' 3 4
+   'dog''s' 2 3
+^D
+[Table [Str "Test"] [AlignDefault,AlignDefault,AlignDefault] [0.0,0.0,0.0]
+ [[]
+ ,[Plain [Str "a"]]
+ ,[Plain [Str "b"]]]
+ [[[Plain [Str "cat's"]]
+  ,[Plain [Str "3"]]
+  ,[Plain [Str "4"]]]
+ ,[[Plain [Str "dog's"]]
+  ,[Plain [Str "2"]]
+  ,[Plain [Str "3"]]]]]
+```
+
+```
+% pandoc -f rst -t native
+.. csv-table:: Test
+   :escape: \
+
+   "1","\""
+^D
+[Table [Str "Test"] [AlignDefault,AlignDefault] [0.0,0.0]
+ []
+ [[[Plain [Str "1"]]
+  ,[Plain [Str "\""]]]]]
+```
+