function which will perform reading of text
files from disk, which should take a path as its first argument. reader() function should
return named character vector: elements of vector = documents,
names of the elements = document ids which will be used in DTM construction.
If user doesn't provied names character vector, document ids will be generated as
file_name + line_number (assuming that each line is a document).
character path of directory. All files in the directory will be read.