Consider throwing error when using fragments inlining with some flags

[kamilzych]

Right now, it's possible to set

• keep_style_tags
• keep_at_rules
• perhaps keep_link_tags as well

to true and inline fragments.
These flags are ignored in such case, because there's simply no place in HTML output to insert <style>...</style> tag.

Based on the discussion in this PR, consider setting these flags to true as invalid when inlining fragments and throw an error.