* Support export of multiple notes to pdf files.
* Update in response to review comments.
Find unique filenames when exporting multiple pdfs.
Simplify delay.
* Eliminate redundant retries and fix error message in fs shim unique filename search.
* Make output filenames consistent between single and multi pdf export.
* Remove unnecessary white space.
* Remove legacy sleep in pdf export and add a little more variability to unique filename search.
* Replace linked Note ids by relative paths in MD Exporter.
* Added tests for the MD Exporter.
* Changed fs.readdirSync use for earlier Node version (v8)
In the previous commit the code used fs.readdirSync from Node v10 or
later. But since Joplin still uses v8, I changed the use of
fs.readdirSync to be in line with the earlier api.
* Updated readDirSync use for Node v10, which allows gets folder names too.
* Revert "Updated readDirSync use for Node v10, which allows gets folder names too."
This reverts commit 8f255db120861dd7773d99e1b63f4864d39594cf.
Because the Travis builds still use Node v8. This is fine as well, the
readdirSync returns the filenames in the directory.
* Added reservedNames param to findUniqueFilename
