Add Comment/Description stored procedures and functions

[xbgmsharp]

As per #307 (comment).
the functions table should show the comment of the function like for tables, columns, etc. Right now the comments are not shown anywhere which makes the table not that useful for documentation.

For postgresql the following query should be updated.

https://github.com/k1LoW/tbls/pull/341/files#diff-e36a277bcc301a326320a2ca3b839ee745128b68008e5200c67313804898758fR374

SELECT n.nspname AS schema_name,
p.proname AS specific_name,
TEXT 'FUNCTION',
t.typname AS return_type,
pg_get_function_arguments(p.oid) AS arguments,
d.description AS description
from pg_proc p
LEFT JOIN pg_namespace n ON p.pronamespace = n.oid
LEFT JOIN pg_type t ON n.oid = p.prorettype 
LEFT JOIN pg_description d ON d.objoid = n.oid
WHERE n.nspname NOT IN ('pg_catalog', 'information_schema')

[emmanuelay]

Id like to have a stab at this.