support multiple suffix rule

Addresses #12 (and a separated pull request of #14)

This pull request enables support of multiple suffix rule:
```
rule '.o' => ['.c', '.h'] => sub {
    my ($t, $dst, @srcs) = @_;
    $dst; # 'a.o'
    my $c = $srcs[0]; # 'a.c'
    my $h = $srcs[1]; # 'a.h'
};
```
I removed 'src' attribute and added 'srcs' attribute, which is an incompatible change.
https://github.com/tokuhirom/Daiku/pull/16/files#diff-8d6279e76c62fde2cd51ee7de977c1c5R12