support multiple suffix rule

Addresses #12 (and a separated pull request of #14)

This pull request enable support of multiple suffix rule:
```
rule '.o' => ['.c', '.h'] => sub {
    my ($t, $dst, @srcs) = @_;

    my $c = $srcs[0]; # 'a.c'
    my $h = $srcs[1]; # 'a.h'