Duckduckgo-search

1. DDGS().text() - add new parameter backend:
api - collect data from duckduckgo.com,
html - collect data from html.duckduckgo.com,
lite - collect data lite.duckduckgo.com.
2. DDGS().text(backend='api') and DDGS().images() - add sleep(0.75) after receiving the vqd.
3. CLI - ddgs text - add -b (--backend) parameter.
4. add tests for DDGS().text(backend='html') and DDGS().text(backend='lite').

DDGS().text() - bugfix in next_page finding

DDGS().text() - parsing html with lxml

Bugfix in DDGS().text()

1. get_url() will raise an exception after unsuccessful retries instead of returning None,
2. assert vqd istead of returning none,
3. update requirements,
4. update README.

Bugfix in useragent: remove anti_useragent