Add plugin APIs for intercepting downloaded files

• include/wget/wget.h: Add function declarations
• libwget/plugin.c: Add virtual functions
• configure.ac: Check for fmemopen()
• src/plugin.c: Add implementation
• src/wget_plugin.h: Likewise
• src/wget.c: Add code that calls the implementation
• tests/test-plugin-dummy.c (pluginapi): Add options 'parse-rot13', 'only-rot13', and 'test-pp'
• tests/test-plugin.c: Add tests
• tests/libtest.c: Load files into dynamic memory instead of stack, enables testing with large files

Plugin APIs for intercepting downloaded files have been implemented as per specifications at https://gitlab.com/akash_rawal/wget2/wikis/Stage-3