In this commit, we add a new function, `ScriptTemplate` to make the
process of making custom Bitcoin scripts a bit less verbose.
ScriptTemplate processes a script template with parameters and returns the
corresponding script bytes. This functions allows Bitcoin scripts to be
created using a DSL-like syntax, based on Go's templating system.
An example of a simple p2pkh template would be:
`OP_DUP OP_HASH160 0x14e8948c7afa71b6e6fad621256474b5959e0305 OP_EQUALVERIFY OP_CHECKSIG`
Strings that have the `0x` prefix are assumed to byte strings to be pushed
ontop of the stack. Integers can be passed as normal. If a value can't be
parsed as an integer, then it's assume that it's a byte slice without the 0x
prefix.
Normal go template operations can be used as well. The params argument
houses paramters to pass into the script, for example a local variable
storing a computed public key.
