Fixes #641 Support fallback fonts

[danfickle]

Fixes #641

Including major refactor of PDF font resolver and passing test.

Why it is required:
Currently fonts not matching the specified font-family in CSS are not used. This PR allows one to register fonts to be used in the event that there are no fonts which match the font-family and support the current character.

Fallback fonts can be registered to try before the built-in fonts (FALLBACK_PRE) or after (FALLBACK_FINAL).

We can now do:

           // DOCUMENT fonts will only match if the `font-family` includes 'SourceSans'.
           builder.useFont(
                    new File("target/test/visual-tests/SourceSansPro-Regular.ttf"),
                    "SourceSans", 400, FontStyle.NORMAL, true,
                    EnumSet.of(FSFontUseCase.DOCUMENT));
            // This font however will be tried after the `font-family` fonts but before the built-in fonts.
            builder.useFont(
                    new File("target/test/visual-tests/Karla-Bold.ttf"),
                    "Karla", 700, FontStyle.NORMAL, true,
                    EnumSet.of(FSFontUseCase.FALLBACK_PRE));

<div style="font-family: 'SourceSans'; font-weight: 400;">
Text should look normal!
</div>

<!-- Using fallback font -->
<div style="font-family: 'no-exist'; font-weight: 400;">
Text should look bold!
</div>

Fallback fonts are sorted from lowest priority to highest. The inputs are provided from CSS.

private int getFontPriority(FontDescription font, String[] families, IdentValue weight, IdentValue desiredStyle, IdentValue variant) {
            String fontFamily = font.getFamily();
            int fontWeight = font.getWeight();
            IdentValue fontStyle = font.getStyle();

            List<String> desiredFamilies = families != null ?
                    Arrays.asList(families) : Collections.emptyList();
            int desiredWeight = FontResolverHelper.convertWeightToInt(weight);

            if (fontWeight == desiredWeight &&
                fontStyle == desiredStyle) {
                // Exact match for weight and style.
                return getFamilyPriority(fontFamily, desiredFamilies);
            } else if (Math.abs(fontWeight - desiredWeight) < 200 &&
                       fontStyle == desiredStyle) {
                // Near enough weight match, exact style match.
                return 3 + getFamilyPriority(fontFamily, desiredFamilies);
            } else if (fontStyle == desiredStyle) {
                // No weight match, but style matches.
                return 6 + getFamilyPriority(fontFamily, desiredFamilies);
            } else {
                // Neither weight nor style matches.
                return 9 + getFamilyPriority(fontFamily, desiredFamilies);
            }
        }

        private int getFamilyPriority(String fontFamily, List<String> desiredFamilies) {
            if (!desiredFamilies.isEmpty() &&
                desiredFamilies.get(0).equals(fontFamily)) {
                return 1;
            } else if (desiredFamilies.contains(fontFamily)) {
                return 2;
            } else {
                return 3;
            }
        }