avoid errors with images embedded in comments

[zkamvar]

In this PR, I have

• added two tests to reproduce HTML comment with image throws an error  #130
• fixed the test with two lines in get_images()
• updated the documentation for get_images() with an example
• updated the NEWS

Testing

You can test this version of {pegboard} by using the pull request helpers from the {usethis} package inside your local clone of {pegboard}

# FETCH AND LOAD -----------------------------------------
library("usethis")
library("devtools")
library("testthat")
pr_fetch(131)
load_all()

# reprex from 130 ----------------------------------------
tmp <- withr::local_tempfile()
txt <- r"{
some text and 

<!-- <img src='whoops.png'> -->
}"
writeLines(txt, tmp)

# Test that it's silent with testthat --------------------
expect_silent(pegboard::Episode$new(tmp)$validate_links())

When you run the above, you should see no output, which means that you have confirmed that #130 is fixed! When you are done testing, you can clean up your workspace by running:

pr_finish()

Background

There was a situation where an HTML image embedded in an HTML comment would produce an error like this using the $validate_links() method:

Error in `purrr::map()`:
ℹ In index: 10.
ℹ With name: 6_Control_flow.md.
Caused by error in `purrr::map()`:
ℹ In index: 1.
Caused by error in `UseMethod()`:
! nicht anwendbare Methode für 'xml_find_all' auf Objekt der Klasse "xml_document" angewendet

This is happening because something like this:

<!-- <img src='hi.png'> -->

is detected as an images within an HTML block on this line in get_images():

pegboard/R/get_images.R

Line 11 in dbc4028

hblock <- ".//md:html_block[contains(text(), '<img')]"

This PR fixes that by explicitly adding the XPath predicate [not(starts-with(normalize-space(text()), '<!--'))] to the statement, which will prevent any HTML starting with <!-- to pass through.

This will fix #130

[ErinBecker]

Thanks for these detailed instructions @zkamvar. I have tested and confirm that the fix works. Merging!