Provide a generalised mechanism for passing command line options 'forward'

[josephwright]

Currently, the core l3build functions pass on -e, etc. when calling scripts in subdirectories. However, that only works if one uses the standard main() function and so on. To allow customised use of the code, some 'forwarding' function would be good. This would help with our own code as we need a custom top-level script and one in the 2e repo.