Ticker feature implementation.
Added new options, such as: "ticker", "ticker_delimeter",
"ticker_interval", "ticker_chars_limit" & "ticker_direction".

"ticker" option enables/disables feature for a given block.
"ticker_delimeter" sets a character to separate tail and head of the
output string.
"ticker_interval" determines the amount of time periods on which the
output string is being shifted.
"ticker_chars_limit" defines the amount of symbols to be displayed.
"ticker_direction" defines the direction in which the characters will
shift.

This feature required to add a library to work with UTF8 characters, so
now i3blocks has "libutf8proc" in its dependencies.

Closes vivien#466

Treat the case when the ticker interval is less than the block interval
in the same manner as when the block interval is set to 'once'.