[MRG] ENH: print_dir_tree can now return a str instead of only printing

[sappelhoff]

closes #590

This enhances print_dir_tree to optionally (default False) return a str instead of only printing.

We can then use this to

1. get a bids dir tree as a str
2. plot that tree using mpl's plt.text
3. set that plot as a thumbnail in the examples.

These three steps could be done either:

• as part of an example --> would be a bit clunky, admittedly
• during the docs build process --> would require a small python snippet that runs in the back to generate the figures and make sure the examples have the paths to those figures to set as thumbnails.

WDYT?

it's a treatment to make these thumbnails more meaningful:

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"

[codecov-io]

Codecov Report

Merging #600 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
+ Coverage   92.72%   92.74%   +0.02%     
==========================================
  Files          17       17              
  Lines        2294     2302       +8     
==========================================
+ Hits         2127     2135       +8     
  Misses        167      167              

Impacted Files	Coverage Δ
mne_bids/path.py	96.36% <100.00%> (+0.05%)	⬆️

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[agramfort]

what's new entry?

[hoechenberger]

Thanks @sappelhoff!