[Bugfix] Fix prepare_wikipedia

[sxjscience]

Description

• [x] wikiextractor is now available in pip so I added it as an external dependency.
• [x] Add the test case about data cli.
• [x] Improve the documentation.

Checklist

Essentials

• [x] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [x] Changes are complete (i.e. I finished coding on this PR)
• [x] All changes have test coverage
• [x] Code is well-documented

cc @dmlc/gluon-nlp-team

[sxjscience]

Fix https://github.com/dmlc/gluon-nlp/issues/1416

FYI @preeyank5

[codecov[bot]]

Codecov Report

Merging #1417 into master will decrease coverage by 0.62%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1417      +/-   ##
==========================================
- Coverage   85.24%   84.62%   -0.63%     
==========================================
  Files          53       53              
  Lines        6954     6868      -86     
==========================================
- Hits         5928     5812     -116     
- Misses       1026     1056      +30     

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
src/gluonnlp/utils/lazy_imports.py	57.95% <66.66%> (-1.81%)	:arrow_down:
conftest.py	76.31% <0.00%> (-9.94%)	:arrow_down:
src/gluonnlp/data/loading.py	75.75% <0.00%> (-7.64%)	:arrow_down:
src/gluonnlp/utils/misc.py	58.25% <0.00%> (-0.95%)	:arrow_down:
src/gluonnlp/data/tokenizers/spacy.py	65.33% <0.00%> (-0.91%)	:arrow_down:
src/gluonnlp/data/tokenizers/huggingface.py	72.44% <0.00%> (-0.47%)	:arrow_down:
src/gluonnlp/models/transformer_xl.py	82.00% <0.00%> (-0.45%)	:arrow_down:
src/gluonnlp/data/tokenizers/jieba.py	73.13% <0.00%> (-0.40%)	:arrow_down:
... and 18 more

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