Add support for indexing with tuples to `placeholder`

[ivan-bocharov]

Currently placeholder function only accepts integer indices. It is convenient to index multidimensional data column-wise or row-wise (see e.g. #17).

An example that will index data column-wise:

for t = 1:T
    ...
    placeholder(y[t], :y, dims=(obs_dim,), index=(:, t))
    ...
end