Prevent additional white space in font credits in readme.txt

[mikachan]

When the font credits are added to the readme.txt file, they add an unnecessary line break above the font license info. This PR should fix this so that a new line break is only added when the readme.txt does not already have a line break at the end of the file. This line break above the font credits block should also be removed from the readme.txt when a font family is removed from the theme.

Testing instructions:

1. Go to the Manage Theme Fonts page
2. Add a new font
3. Check that the font license info has been added to the theme readme.txt and that there is a line break between the previous end of the readme contents and the new font credits
4. Go back to the Manage Themes Fonts page
5. Remove a font family
6. Check that the font license info has been removed from the readme.txt and there are no unnecessary line breaks left behind

Closes #361.

[vcanales]

Works as advertised; LGTM.