Add optional parameter for generating file with encrypted tags to an output file

[calvn]

Right now the output is simply printed, which is fine for encrypting a single tag. However, for encrypting larger files that contains both encrypted tags and plaintext data, handling file creation from within gosecret is more ideal than redirecting its output to a file (similar to what consul-replicate does).

This is not a pressing feature since this function can be done via stdout redirect, but nevertheless suggested as an enhancement.