Feature request: a way to define which characters shouldn't get escaped/encoded

[veqryn]

This library hard codes the "unsafe" characters as:

my $unsafe = '^A-Za-z0-9\-\._' . $safe;

Then it allows you to add the following as additional safe characters, if you have + in your template key: ```perl
my $RESERVED = q(:/?#[]@!$&'()*+,;=);

In some situations I wish to add `~` to the safe list, but no other characters.  
In other situations I also want to add `/` to the safe list.  
In most cases, I do not want the full reserved list.

Feature request: a way to define which characters shouldn't get escaped/encoded