mergeWith option doesn't work due to failing file_exists() check

[ocean90]

Issue Overview
Traduttore has a handy mergeWith option which allows to merge an existing POT file with the default generated by Traduttore. Unfortunately this option doesn't work

To Reproduce
Steps to reproduce the behavior:

1. Create a project with a custom POT file and the mergeWith option
2. Let Traduttore import the project
3. After import notice that the strings from the custom POT file are not there

Expected behavior
Strings from the custom POT file are also imported.

Additional context

traduttore/inc/Updater.php

Lines 186 to 188 in 413cff4

	if ( $merge && ! file_exists( $merge ) ) {
	$merge = null;
	}

The file_exists() is used for a relative path while the current directory may not be the directory of the plugin, usually the root directory of WordPress itself. The fix would be to prefix the merge value with $source.