Fix the case when result of map-lookup is falsy (#114)

* Fix the case when result of map-lookup is falsy

* Add test

Add PARSE.DATE and PARSE.DATETIME functions (#112)

Add two new function: `PARSE.DATE()` and `PARSE.DATETIME()`

The first argument is a date/time string, second - formatter string as described in section "Patterns for Formatting and Parsing" of [that article](https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html)

All alpha-characters in formatter string should be quoted using `'` character, eg. `"YYY'T'MM:ss"`

Add REGEX function to compile a string to pattern (#111)

Accept the second argument as a string of supported flags:

  * "i" Makes the expression search case-insensitively.
  * "s" dot all. It makes a wild character `.` match newlines as well.
  * "m" Makes the boundary characters `^` and `$` match the beginning
        and ending of every single line instead of the beginning and
        ending of the whole string.

Combination of flags supported via the concatenation of characters: "si"

Fix setting and environment variable

Fixes for autocomplete function (#110)

Remove codecov

Use unified excel-number function for to-integer and to-float coercion (

#108)

fix empty object case

Use stable version of matchete

Temporary disable cache