Fix file corruption when strings count is too large

The previous implementation only worked with less than 9999999 strings
in total. Having more strings than this number caused the sharedStrings.xml
file to become corrupted (because valid characters were overwritten).

=> Increased the default number to 9999999999999
=> Refactored the way the replacement of the default count is done
Merge pull request #3 from adrilo/fix_strings_count_corruption_issue

Fix file corruption when strings count is too large