Process pseudo XML tags more properly

[artlomov]

Allow multiline tags. Process true and false attribute values as boolean, not as integer.

Valid tags examples:

<magick resize="600" background="Orange label:'Scheme 1' +swap" gravity="Center" append="append" output_format="jpg">
![Картинка](_img/pic.eps)
</magick>

<magick resize="600"
background="Orange label:'Scheme 2' +swap" gravity="Center"
append="True" output_format="jpg">
![Картинка](_img/pic.eps)
</magick>

<magick
command_params="-resize 600 \
-background Orange label:'Scheme 3' +swap \
-gravity Center \
-append"
output_format="jpg">
![Картинка](_img/pic.eps)
</magick>

[artlomov]

One more commit to get closer to XML spec. Allow attribute names like this:

_test-attr:ibute.019

Attributes with hyphens are needed for foliantcontrib.imagemagick preprocessor.