Add some new functions to extension c_functions.

Add some new functions to extension c_functions.
New functions include oils_xslt_process, vandelay.add_field and vandelay.strip_field. These functions are implemente in plperlu before.
The C implementation of these functions relies on ICU4C and libxml2, libxslt. To install the new functions just create the extension
c_functions as before.
All the functions have not tested yet. There may be some errors in it. I will test it soon.

Signed-off-by: Swenyu Duan <dsy@sina.com>