Support builtin types as field processors

[Gallaecio]

Found by @vmruiz.

[codecov[bot]]

Codecov Report

Merging #172 (f335491) into master (bfd6683) will increase coverage by 0.00%. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #172 +/- ## ======================================= Coverage 98.59% 98.60% ======================================= Files 30 30 Lines 1356 1362 +6 ======================================= + Hits 1337 1343 +6 Misses 19 19 ``` | [Impacted Files](https://app.codecov.io/gh/scrapinghub/web-poet/pull/172?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub) | Coverage Δ | | |---|---|---| | [web\_poet/fields.py](https://app.codecov.io/gh/scrapinghub/web-poet/pull/172?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-d2ViX3BvZXQvZmllbGRzLnB5) | `99.11% <100.00%> (ø)` | | | [web\_poet/utils.py](https://app.codecov.io/gh/scrapinghub/web-poet/pull/172?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-d2ViX3BvZXQvdXRpbHMucHk=) | `100.00% <100.00%> (ø)` | |

[kmike]

Thanks @Gallaecio!