Add sensible default minimum_should_match

Number of | Minimum
optional  | should
clauses   | match
----------+---------
1         | 1
2         | 2
3         | 2
4         | 3
5         | 3
6         | 3
7         | 3
8+        | 50%

The setting was worked out by using the comparison feature of bin/search
with various example queries of different lengths (3, 4, 5, 7, 9 words)
and inspecting the consequences on search results.