add page margin feature

[antoniolucasnobar]

provide a way to user define page margins: maybe passing an options object to the convert method.

to think about: Is it possible to check the outer most element has the border attribute, and then use its values?

the margin values in RTF:
{\pgdsc0\pgdscuse451\pgwsxn12240\pghsxn15840\marglsxn1800\margrsxn1800\margtsxn3141\margbsxn3708\pgdscnxt0 Default Style;}}
\formshade{*\pgdscno0}\paperh15840\paperw12240\margl1800\margr1800\margt1440\margb1440\sectd\sbknone\sectunlocked1\pgndec\pgwsxn12240\pghsxn15840\marglsxn1800\margrsxn1800\margtsxn3141\margbsxn3708\ftnbj\ftnstart1\ftnrstcont\ftnnar\aenddoc\aftnrstcont\aftnstart1\aftnnrlc

From RTF docs:

• \marglN Left margin in twips (default is 1800).
• \margrNRight margin in twips (default is 1800).
• \margtNTop margin in twips (default is 1440).
• \margbNBottom margin in twips (default is 1440).

check page 49 of https://web.archive.org/web/20190708132914/http://www.kleinlercher.at/tools/Windows_Protocols/Word2007RTFSpec9.pdf