ENH: extract-text now can store output in files, each page's text can be stored in a different file

[papametis]

This enhancement adds an option --output-pattern to extract-text which allows it to store its output in files instead of echoing it, either the entire output in a single file, or each page's text in a different file, depending on the output pattern given.

Also, since there were no unit tests for the extract-text command before, test-extract-text.py was made with 2 unit tests for the command (one for an optionless use of extract-text, where the output is echoed, and one where --output-pattern is used to create files with the output).

Fixes #213 .

Checklist:

• the code of the new feature
• a unit test covering its behaviour
• a mention of this addition in CHANGELOG.md
• a usage example and documentation about it in docs/user/
• this PR is ready to be merged

By submitting this pull request, I confirm that my contribution is made under the terms of the BSD 3-Clause license.