Period marker only works with Date object

The `plot` function only plots the period marker if the time index is a `Date` object, and fails to plot for `'DateTime`/`POSIX` index (e.g. 2017-02-02 03:00).

It seems concatenating a `POSIX` timestamp with `NULL` (e.g. https://github.com/google/CausalImpact/blob/master/R/impact_plot.R#L82) does not really work

> c(NULL,strptime('2017-02-02 03:00','%Y-%m-%d %H:00'))
$sec
[1] 0

$min
[1] 0

$hour
[1] 3

$mday
[1] 2

$mon
[1] 1

$year
[1] 117

$wday
[1] 4

$yday
[1] 32

$isdst
[1] 0

$zone
[1] "CET"

$gmtoff
[1] NA

whereas `c(NULL,as.Date('2014-02-01'))` returns the expected value.