Support for multiline strings

[relaxolotl]

It'd be great if we could have support for multiline strings. This should help mitigate validation issues caused by ICU syntax and XML tags.

The strings will also be much more readable especially for pluralized and gendered strings. Something like the below example would be fairly unreadable if it was compressed into a single line.

`{gender_of_host, select, 
   female {
     {num_guests, plural, offset:1 
       =0 {{host} does not give a party.}
       =1 {{host} invites {guest} to her party.}
       =2 {{host} invites {guest} and one other person to her party.}
       other {{host} invites {guest} and # other people to her party.}}}
   male {
     {num_guests, plural, offset:1 
       =0 {{host} does not give a party.}
       =1 {{host} invites {guest} to his party.}
       =2 {{host} invites {guest} and one other person to his party.}
       other {{host} invites {guest} and # other people to his party.}}}
   other {
     {num_guests, plural, offset:1 
       =0 {{host} does not give a party.}
       =1 {{host} invites {guest} to their party.}
       =2 {{host} invites {guest} and one other person to their party.}
       other {{host} invites {guest} and # other people to their party.}}}}
`

[mitchchn]

@relaxolotl in what sense aren't these supported already?