KString convert to relative time func

[maxwroc]

Currently relative time is being shown only in case when the KString has only single dynamic value and that value is a date.

With this change we will be able to add relative time values in more complex strings e.g.

secondary_info: "{last_changed|reltime()}, Type: {sensor.my_deivice_battery_type.state}"

In the above example two dynamic values were used together with the static text, where the first dynamic value is being converted to relative time (this was not possible before)

#608

[coveralls]

Pull Request Test Coverage Report for Build 7293311474

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+3.2%) to 83.853%

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Totals
Change from base Build 7278642938:	3.2%
Covered Lines:	350
Relevant Lines:	400

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💛 - Coveralls