Bug fixed and new tag added

[Gareth]

Thanks to Marcin for the feature request and bug report:-

1. Fixed repeat tag which was locking up when second param was not
supplied.
2. Added new str_insert tag to allow a string to be inserted at a
specified position

param1 - string to be inserted
param2 - position to be inserted
param3 - 0 || 1 if the string insertion happens at the beginning or
end of the character

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