Markdown tables: implement table_attributes extension.

When `table_attributes` is enabled (as it is by default for
pandoc's Markdown), attributes can be attached to a table by
including them at the end of the caption. Previously the writer
would emit an identifier in this position, but the reader didn't
handle it. Now arbitrary attributes are allowed, and they work in
both the reader and writer.

Closes #10884.

[API change]: Text.Pandoc.Extensions: Add `Ext_table_attributes`
constructor for `Extension`.

test/command/10884.md

@@ -0,0 +1,17 @@
+```
+% pandoc -f markdown -t markdown
+  header1   header2
+  --------- ---------
+  r1c1      r1c2
+  r2c1      r2c2
+
+  : mycaption {#myid .myclass key="value"}
+^D
+  header1   header2
+  --------- ---------
+  r1c1      r1c2
+  r2c1      r2c2
+
+  : mycaption {#myid .myclass key="value"}
+
+```

test/tables/nordics.markdown

@@ -15,4 +15,4 @@
            Total                                  27,376,022      1,258,336
   -------------------------------------------------------------------------------
 
-  : States belonging to the *Nordics.* {#nordics}
+  : States belonging to the *Nordics.* {#nordics source="wikipedia"}

test/tables/students.markdown

@@ -20,4 +20,4 @@
 | 4100332                                 | Toyota, Hiroko                          |
 +-----------------------------------------+-----------------------------------------+
 
-: List of Students {#students}
+: List of Students {#students source="mdn"}