adding new functionality suggested in #107

[T-Engel]

locate_columns() allows users to manually locate separators between columns. It adopts the approach of the "reduced" widget from the related function locate_areas(). The output can be used as the columns argument in extract_tables().
#107

Please ensure the following before submitting a PR:

• if suggesting code changes or improvements, open an issue first
• for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
• add code or new test files to /tests for any new functionality or bug fix.
• make sure R CMD check runs without error before submitting the PR

I am not familiar with test files. Could anyone help me here? How do I test an interactive function?

[T-Engel]

I don't understand why build job 110.5 failed on travis. Any ideas?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7723d2b). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #108   +/-   ##
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  Coverage          ?   52.25%           
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  Files             ?       13           
  Lines             ?      666           
  Branches          ?        0           
=========================================
  Hits              ?      348           
  Misses            ?      318           
  Partials          ?        0

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[pachadotdev]

let me test how these changes go with tabula 1.0.5