Support including or excluding list of schemas, not just single schema

[daa]

Work done in #58 adds an ability to include or exclude single schema, but recently I've met the need to limit considered schemas to a list of them or symmetrically to exclude a list of schemas from consideration, hence this feature request. Also I'd like to submit a pull request with implementation.

As for implementation I see 2 possible ways:

• make get_inspector() and schemainspect.pg.obj.PostgreSQL.filter_schema() to accept list of strings in schema and exclude_schema parameter. Here again we have 2 possibilities: accept list in addition to just string to keep old behaviour or accept only list and break compatibility
• treat schema and include_schema parameters as regular expression patterns thus keeping function signature but slighly changing old behaviour: passing plain string would mean matching schema by given prefix

What do you think? Personally I lean towards the first alternative, i.e. accept a list of strings in addition to just string in corresponding function and method parameters.

[daa]

Recently I thought that we have a third alternative: make database inspector and consequently get_inspector() accept argument with predicate function to match schema and then apply this function to filter affected schemas. We may reuse schema argument or add a new argument, for example accept_schema in addition to schema and exclude_schema and may be gradually deprecate the latter because proposed approach covers all their possible usage variants. I'd like to hear your opinion on this.

[actuchicks]

Thanks for raising this @daa - we also have a need to exclude multiple schemas from migra processes and would be really keen to see this resolved 🙌