Allow to use custom-defined statements in extended_vars

I'd like to add several 'syn match' on per-project basis - widely used helper subs, for example.
But they are not count inside of extended vars properly.

Here is quick example:

```
        " internal functions:
        " 1. start wiht _, _then_only_lower_case
        " 2. have at least two "words" separated by _, only lower case
        syn match perlCustomStatement1  "\<\%(_[a-z0-9_]\+\)\>\%((\?\)\@="
        syn match perlCustomStatement1  "\<\%([a-z0-9]\+_[a-z0-9_]\+\)\>\%((\?\)\@="
```

Later in perl code such names are not highlighted properly:
```perl
my %hash = %{ _get_some_hash() };
```

Proposed fix allow to define up to 10 custom syntax rules that will be highlighted properly.