implement POSIX regexp support

implement POSIX regexp support

Introduce a new operator `~` and new `/.../eis` regular expression syntax.

This allows filtering by regular expression, e.g.

   jsonfilter -s '[ "foo", "bar", "baz" ]' -e '$[@ ~ /^b/]'

... would yield the values `bar` and `baz`.

Possible regular expression modifiers are:

  - `e` ... enable extended POSIX regular expressions
  - `i` ... perform case insensitive matches
  - `s` ... let ranges and `.` match the newline character

A regular expression literal may occur on the left or the right side of
the `~` operator, but not on both.

In case neither side of the `~` operator is a regular expression, the right
side will be treated as regular expression pattern. Non-string values are
converted to their string representation before performing matching.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>