Multiple browser contexts

[elacuesta]

Closes #12

Tasks:

• Multiple contexts at startup
• Create new contexts during a crawl
• Example spider
• Settings docs
• Usage docs
• Tests
• Deprecate PLAYWRIGHT_CONTEXT_ARGS in favour of PLAYWRIGHT_CONTEXTS

[codecov]

Codecov Report

Merging #13 (1f51309) into master (68d8fd1) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 1f51309 differs from pull request most recent head 9c4c687. Consider uploading reports for the commit 9c4c687 to get more accurate results

@@            Coverage Diff            @@
##            master       #13   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          136       159   +23     
=========================================
+ Hits           136       159   +23     

Impacted Files	Coverage Δ
scrapy_playwright/handler.py	100.00% <100.00%> (ø)

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[elacuesta]

@kalessin could you take a look at this? Thanks!