Filtering support

[mcollina]

Things to do:

• Support filtering in get() and getStream().
• Support filtering in search() and searchStream().
• Support async filtering of search solutions.
• Update docs.

Closes #27.

[coveralls]

Coverage remained the same when pulling c719afb on filtering into 6af24a3 on v0.7-wip.

[coveralls]

Coverage remained the same when pulling a05de43 on filtering into 6af24a3 on v0.7-wip.

[coveralls]

Coverage remained the same when pulling 57a2c8e on filtering into 6af24a3 on v0.7-wip.

[coveralls]

Coverage remained the same when pulling 096e684 on filtering into 6af24a3 on v0.7-wip.

[coveralls]

Coverage remained the same when pulling 2bb8b8e on filtering into 6af24a3 on v0.7-wip.

[coveralls]

Coverage remained the same when pulling 3fd8af0 on filtering into 6af24a3 on v0.7-wip.

[coveralls]

Coverage remained the same when pulling 3fd8af0 on filtering into 6af24a3 on v0.7-wip.