Add support for XML formatting

[BoboTiG]

Slightly inspired from #720, it explains the credits.
Closes #825.

XML test files are from the defused repository.

As a side effect, XHTML responses will be pretty-printed too.

[codecov-commenter]

Codecov Report

Merging #1129 (d7deb04) into master (8618f12) will increase coverage by 0.00%.
The diff coverage is 97.26%.

❗ Current head d7deb04 differs from pull request most recent head 65a1768. Consider uploading reports for the commit 65a1768 to get more accurate results

@@           Coverage Diff           @@
##           master    #1129   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files          65       67    +2     
  Lines        4170     4243   +73     
=======================================
+ Hits         4052     4123   +71     
- Misses        118      120    +2     

Impacted Files	Coverage Δ
httpie/cli/constants.py	100.00% <ø> (ø)
tests/test_output.py	99.47% <ø> (ø)
httpie/output/formatters/xml.py	97.05% <97.05%> (ø)
tests/test_xml.py	97.14% <97.14%> (ø)
httpie/plugins/registry.py	100.00% <100.00%> (ø)
tests/fixtures/__init__.py	100.00% <100.00%> (ø)

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