More XLSX methods

[nikolai-b]

Would you be open to extracting/exposing a few more stylistic things such as borders and cell alignment? I might be able to find the time to do this but wanted to check if it was a direction you were open to first.

At the moment I can do things via

roo.styles.instance_variable_get(:@doc)

so even exposing the WeakRef doc would make it possible.

Thanks