LP writer creating invalid names for COIN CBC

[d3netxer]

In my Julia file I am creating variables. The output .lp file has brackets for the variables and is creating many errors like the following when I am running cbc from the command line:

### CoinLpIO::is_invalid_name(): Name x[1] contains illegal character '['

Is there any way to fix this? Maybe we can replace the brackets with underscores?

[odow]

LPWriter.jl has some logic about handling valid names. See https://github.com/odow/LPWriter.jl/blob/master/src/LPWriter.jl

We should probably do something similar. I haven't finished #32

[odow]

Closed by #71