FilesystemFdw: Enhance pattern matching and timestamp support

[rvernica]

Highlights:

• Support regular expressions in pattern option
• Allow for case sensitive or insensitive pattern matches
• Expose file mtime and ctime as timestamps

This PR adds four more options to FilesystemFdw:

• escape_pattern (default: TRUE)
  If TRUE, the pattern used to match files is escaped before it is
  used for regular expression matching. If FALSE, the pattern used to
  match files is used as is and it is assumed to be a valid regular
  expression.
• ignore_case (default: FALSE)
  If FALSE, the pattern used to match files is case sensitive. If
  TRUE, the pattern used to match files is case insensitive.
• mtime_column
  If set, defines which column will contain the file mtime.
• ctime_column
  If set, defines which column will contain the file ctime.

With this PR, the following example is supported:

> ls -R1 f
f:
taz

f/taz:
a_b.jpeg
a_b.JPEG
a-b.jpg
a_b.png
a b.PNG

CREATE FOREIGN TABLE foo (
    filename VARCHAR,
    mtime TIMESTAMP,
    ctime TIMESTAMP,
    foo VARCHAR,
    bar VARCHAR,
    taz VARCHAR
) SERVER filesystem_srv OPTIONS (
    root_dir        '/f',
    pattern         '{taz}/{foo}[ _-]{bar}\.(jpe?g|png)',
    escape_pattern  'FALSE',
    ignore_case     'TRUE',
    filename_column 'filename',
    mtime_column    'mtime',
    ctime_column    'ctime');
SELECT * FROM foo;

 filename |        mtime        |        ctime        | foo | bar | taz 
----------+---------------------+---------------------+-----+-----+-----
 a_b.jpeg | 2018-04-19 18:37:06 | 2018-04-20 19:20:12 | a   | b   | taz
 a_b.png  | 2018-04-19 18:37:04 | 2018-04-20 19:20:12 | a   | b   | taz
 a-b.jpg  | 2018-04-19 18:37:30 | 2018-04-20 19:20:12 | a   | b   | taz
 a_b.JPEG | 2018-04-19 18:37:09 | 2018-04-20 19:20:12 | a   | b   | taz
 a b.PNG  | 2018-04-19 18:37:15 | 2018-04-20 19:20:12 | a   | b   | taz
(5 rows)

Notice the regular expression used for the pattern option which allows for the foo and bar tokens to be separated by , _, or -. Also, the file extensions can be .jpg, .jpeg, or .png, case insensitive.

Fix for #203