Add new --regex option to set regular expression used to find words

A new command line option -r / --regex is implemented to allow the user
to set the regular expression which is used to find the words in the
input lines. This is useful in projects where you want to overwrite the
default expression, for example to split words after an underscore like
in ID_ESTABLISED.
The new regex option cannot be used together with the --write-changes
functionality since it is not guarantied that this will not break code.