Read and write FoLiA XML file using iterparse

The add_liwc_entities script was updated to read the FoLiA XML file
using lxml.iterparse and write the contents to a new file. The next
steps are to add an entity-annotation tag, and to add the entity tags.
Add entity-annotation element

An entity-annotation element is added to the output xml file specifying
the liwc entity annotation. For some reason, this added element is not
properly pretty printed, this must be fixed later.
Add entity elements for LIWC entities

Entity elements are added to the output FoLiA file for all LIWC entities
that occur in the text.
Add annotation element using SubElement factory

The annotation elements is added to the XML using the etree.SubElement
factory. This is cleaner code.
Fix pretty printing output XML

Although the option 'pretty_print' was set when writing the output to
file, the output was not pretty. This issue was fixed by setting the
'remove_blank_text' parameter when loading the input file.
Added argument for output directory

The add_liwc_entities script now takes an additional argument that
specifies the directory that is used to write the output file to. This
additional argument will be used by the shell script for batch adding
LIWC entities to FoLiA files. The output files are saved to a different
directory so that the original files are not changed and do not get
mixed up with the FoLiA that contain the LIWC entities.
Update documentation of add_liwc_entities script

The documentation of the add_liwc_entities script was updated. It now
states that the script takes an additional argument that specifies the
output directory.
Add shell script to add LIWC entities to a dir of FoLiA files

Added a shell script that runs the add_liwc_entities script for a
directory that contains FoLiA files and writes the output files to
another directory.
Merge branch 'feature/batch-add-liwc-entities' into develop
Print document counter when storing FoLiA in ElasticSearch

A counter indicating how many files already have been processed by the
folia2es script is printed together with the file name of the file
currently being processed. This makes it easier to estimate how long it
will take before all files have been stored in ElasticSearch.