Add a C ifdef preprocessor to the stellar-xdr cli

[sisuresh]

From @leighmcculloch here

Add a simple .x C ifdef preprocessor to the stellar-xdr cli that'll accept a .x file and a list of features to enable and output the .x file with ifdef's removed based on the features listed that should be enabled. Platform's projects will use this to preprocess .x files prior to codegen so that they control what features are included in codegen.

[leighmcculloch]

Suggested use:

$ cat file.x | stellar-xdr xfile preprocess --features feat1,feat2
[file.x with feat1 and feat2 enabled, all other feats disabled]