Add support for text alignment in paragraphs

[r-Larch]

This PR enables use cases like the following.

Feel free to merge or close 👍

kind regards
René

[Fact]
public void CanHandleTextAlignment() {
    AssertSuccessfulConversion(
        ConvertToHtml("text-align.docx", mammoth => mammoth
            .AddStyleMap("p[text-align='center'] => p.center:fresh")
            .AddStyleMap("p[text-align='right'] => p.right:fresh")
            .AddStyleMap("p[text-align='justify'] => p.justify:fresh")
        ),
        "<p class=\"center\"><strong>The Sunset Tree</strong></p>" +
        "<p class=\"justify\">Lorem ipsum justify</p>" +
        "<p class=\"justify\">Foobar justify</p>" +
        "<p class=\"right\">TextAlign Right</p>" +
        "<p class=\"center\">Center 1</p>" +
        "<p class=\"center\">Center 2</p>" +
        "<p>Left 1</p>" +
        "<p>Left 2</p>");
}

[Fact]
public void CanHandleTextAlignmentWithStyle() {
    AssertSuccessfulConversion(
        ConvertToHtml("text-align_mixed.docx", mammoth => mammoth
            .AddStyleMap("p[text-align='center'] => p.center:fresh")
            .AddStyleMap("p[text-align='right'] => p.right:fresh")
            .AddStyleMap("p[text-align='justify'] => p.justify:fresh")
            .AddStyleMap("p[style-name='Heading 1', text-align='center'] => h1.center:fresh")
            .AddStyleMap("p[style-name='Heading 1', text-align='right'] => h1.right:fresh")
        ),
        "<h1 class=\"center\">Header 1 Center</h1>" +
        "<h1>Header 1 Left</h1>" +
        "<h1 class=\"right\">Header 1 Right</h1>" +
        "<p>Normal paragraph, lorem ipsum.</p>" +
        "<p class=\"center\">Normal paragraph, align center, lorem ipsum.</p>");
}