aacenc: add AAC_CODER_(FAAC|ANMR|etc.) macros

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
aacenc: use constants to set AAC coder

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
doc/encoders: add doc for AAC encoder

Thanks-to: Kostya Shishkov <kostya.shishkov@gmail.com>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
doc/encoders: Improve libvo-aacenc doc

Signed-off-by: Timothy Gu <timothygu99@gmail.com>