Add get_meta_value

[gavinclive]

Some meta tag may use value instead of content property

Added get_meta_value to handle this kind of case, also when both content nor value property is present, will return null

Example case of meta value property: Medium read time meta data

<meta data-rh="true" name="twitter:label1" value="Reading time" />
<meta data-rh="true" name="twitter:data1" value="3 min read" />

[shweshi]

Thanks @gavinclive