Imaginary numbers literals in sexp templates

Due to `text/scanner` limitations, it's currently impossible to match imaginary numbers.
Probably should use `go/scanner` or extend `text/scanner` in this regard.

Tests that should pass after this is solved:
```go
	{`1 + 1E6i`, `(+ 1 $imag)`},
	{`1 + 1E6i`, `(+ $int 1E6i)`},
	{`1 + 1E6i`, `(+ $int $imag)`},
```