Add engine environment flags 'HORS_ENGINE'

[dzervas]

The only consistent change that I'd think of doing to the program (configuration-wise) is the default engine - to avoid passing -e every time, so I added env var support with the HORS_ENGINE variable.

Example (I found a query that was different for duckduckgo vs google):

dzervas hors> ./target/debug/hors -e google rust vec string join chinese version wow
- Answer from https://stackoverflow.com/questions/7488206
SELECT id, CONCAT(firstName, ' ', lastName) AS fullName FROM table

dzervas hors> ./target/debug/hors -e duckduckgo rust vec string join chinese version wow
Search for target link failed: Parse("Can\'t find search result...")

dzervas hors> HORS_ENGINE=google ./target/debug/hors rust vec string join chinese version wow
- Answer from https://stackoverflow.com/questions/7488206
SELECT id, CONCAT(firstName, ' ', lastName) AS fullName FROM table

[dzervas]

(BTW duckduckgo is for some reason inconsistent, sometimes the test_search_links_with_duckduckgo_search_engine test fails with panicked at 'called `Result::unwrap()` on an `Err` value: Parse("Can\'t find search result...")', tests/test_engine.rs:57:6 for an unknown to me reason)

[WindSoilder]

Hi, thanks for your contribution :-D It's excellent work!

The origin howdoi program also have something like HOWDOI_SEARCH_ENGINE environment variable. And we can also have this :) Thanks for your idea

[WindSoilder]

(BTW duckduckgo is for some reason inconsistent, sometimes the test_search_links_with_duckduckgo_search_engine test fails with panicked at 'called `Result::unwrap()` on an `Err` value: Parse("Can\'t find search result...")', tests/test_engine.rs:57:6 for an unknown to me reason)

Oh yeah, for this problem, I have submit a new issue.

[dzervas]

maybe we should change it to HORS_SEARCH_ENGINE to be a bit more friendly to old howdoi users?

[WindSoilder]

Hmm, I'm ok with both option. Because HORS_ENGINE seems more simpler