Implement xml preprocessing

[halex2005]

Could make preprocessing for xml:

• include xpath reference to current place - substitute current <include xpath="" /> with xml elements that matches to specified xpath

  <some>
    <reference>reference</reference>
  </some>
  <other>
    <include xpath="/some/reference" />
  </other>

  will produce after preprocessing:

  <some>
    <reference>reference</reference>
  </some>
  <other>
    <reference>reference</reference>
  </other>

• More generally, include xpath from file specified - <include xpath="" file="" />. If xpath attribute is absent - include root xml element. If file attribute is absent - use xpath for current file.

Optional: file attribute can contains environment variables that needed to be substituted.