Just extraction

[davidhyman]

Adds a '-x', '--export' option to Yotta Salient points:

• find/replace: top level module directories are renamed on export, I've given them a variable and called them 'vendor_*' when exporting
• find/replace: yotta module build directory likewise (ym -> modules)
• export copies & renames cached module source directories if possible
• find/replace: to allow the export to work, now use relative paths throughout
• also used explicit iteration in the templates where possible, rather than a mix. allows common use of filter function (self.relative)
• need to template the yotta header file as file path needs setting at CMake run time