Allow querying terms with specified taxonomy/taxonomies

[Dartui]

Simple helper for querying terms in specified taxonomy/taxonomies. Usage:

Term::whereTaxonomy('foo')->get();
Term::whereTaxonomies(['foo', 'bar'])->get();

Of course it can be chained to get only one term in specified taxonomy:

Term::whereTaxonomy('categories')->where('slug', 'projects')->first();

Without this code similar or same query could be generated in this way:

 // second where is collection method, not database query
Taxonomy::where('taxonomy', 'categories')->get()
    ->pluck('term')->where('slug', 'projects')->first();

// code copied from this PR
Term::whereHas('taxonomy', function ($query) {
    $query->whereIn('taxonomy', 'category');
})->where('slug', 'projects')->first();

[jgrossi]

@Dartui can you update this PR just to rerun Travis CI please? changed the base branch to 3.0 and would like to double check if tests are passing before merging. thank you