re-Config-uration

[muglug]

This fixes #26 and #21 by changing how the config works, simplifying it while giving users more flexible ways to progressively check their code.

Now, instead of writing

<issueHandlers>
  <PossiblyUndefinedVariable>
    <includeFiles>
      <directory name="src/Core" />
    </includeFiles>
  </PossiblyUndefinedVariable>
</issueHandlers>

you'll write

<issueHandlers>
  <PossiblyUndefinedVariable errorLevel="suppress">
    <errorLevel type="error">
      <directory name="src/Core" />
    </errorLevel>
  </PossiblyUndefinedVariable>
</issueHandlers>

,
which gives you the ability to gradually introduce warnings in other parts of your codebase e.g.

<issueHandlers>
  <PossiblyUndefinedVariable errorLevel="info">
    <errorLevel type="error">
      <directory name="src/Core" />
    </errorLevel>
    <errorLevel type="suppress">
      <directory name="tests" />
    </errorLevel>
  </PossiblyUndefinedVariable>
</issueHandlers>

This also introduces the ability to hide certain subfolders from Psalm's view in the <projectFiles> config:

<projectFiles>
  <directory name="src" />
  <ignoreFiles>
    <directory name="src/ignoreme" />
  </ignoreFiles>
</projectFiles>

cc @erunion @nickyr

[nickyr]

New syntax looks great! Will the old syntax just throw an error now?

[muglug]

@nickyr yup - going to add a formal XSD schema shortly

[erunion]

Dope idea on adding the XSD!