Raise syntax error for unknown s/// flags

[aureliojargas]

If you inform an invalid flags at the end of the s/// command, they are silently removed.

$ sedsed -e 's/foo/bar/h5!' 
s/foo/bar/
$ sedsed -e 's/foo/bar/gX' 
s/foo/bar/g
$ sedsed -e 's/foo/bar/Xg' 
s/foo/bar/
$

Note in the last example that even valid flags (such as g) are removed if they appear after an invalid flag.

An invalid flag is a syntax error and the parser should act accordingly: raise a fatal error.

[aureliojargas]

Fixed by the new sedparse parser, see #52.

$ ./sedsed.py -e 's/foo/bar/h5!' 
ERROR: sedparse: -e expression #1, char 11: unknown option to `s'
$ ./sedsed.py -e 's/foo/bar/gX'
ERROR: sedparse: -e expression #1, char 12: unknown option to `s'
$ ./sedsed.py -e 's/foo/bar/Xg'
ERROR: sedparse: -e expression #1, char 11: unknown option to `s'
$