wasmprinter: make indent character configurable

[oovm]

I find the indentation is not very noticeable when reading nested blocks.

I wish it could be increased to 8 spaces.

Do I just need to modify this place and add the configuration item:

wasm-tools/crates/wasmprinter/src/lib.rs

Line 1416 in 57b1ace

self.result.write_str(" ")?;

[alexcrichton]

Sounds reasonable to me yeah!