Add `LineString` constructors based on `Point`s and `LineString`s

[mathieu17g]

@visr what about adding LineString constructors based on Points and LineStrings?

It could look like:

LineString(ls::LineString, pt::Point) = LineString([coordinates(ls); [coordinates(pt)]])
LineString(ls::LineString, pt1::Point, pt2::Point, pts::Vararg{Point}) = LineString(LineString(ls, pt1), pt2, pts...)
LineString(pt1::Point, pt2::Point) = LineString([coordinates(pt1), coordinates(pt2)])
LineString(pt1::Point, pt2::Point, pt3::Point, pts::Vararg{Point}) = LineString(LineString(pt1, pt2), pt3, pts...)

This may be extended to other geometry types