Be more conservative about attribute escaping

[tkluck]

Attribute escaping breaks e.g. charset declarations (at least in firefox), leading to [workarounds like this(https://github.com/tkluck/StatProfilerHTML.jl/blob/109ad6f/haml/index.hamljl#L4). This issue is also reported by @mlhetland in #6 . We could fix this by e.g. only escaping "; maybe there's authoritative behaviour documented somewhere?