emit does not seek the end of the file before flush

[tseste]

Hey there,

I've been using your package lately and I noticed that before the logger flushes a message,
it is not seeking for the end of the file.

For me it was an issue because I use one more stream to write to the same file, and that other stream actually
handles the seek before writing.

It is really a very small change and it might help others to.

The addition is to use seekend(handler.io) in the beginning of emit function here

Memento.jl/src/handlers.jl

Line 206 in aea16b6

function emit(handler::DefaultHandler{F, O}, rec::Record) where {F<:Formatter, O<:IO}

Let's say line 207

[tseste]

Actually, more like

if typeof(handler.io) <: IOStream
    seekend(handler.io)
end