cct: Add padding support

Whitespaces before @ are now allowed and are copied verbatim to the
output. This allows for "smart" padding when generation source code.

Consider this example:

@ def multiply(in, out)
@     for i in range(0, 4):
{{ out }}[{{ i }}] = {{ in }}[{{ i }}] * coef[{{ i }}]
@     end
@ end

for (i = 0; i < w; i++) {
	@ multiply('R_in', 'R_out')
	@ multiply('G_in', 'G_out')
	@ multiply('B_in', 'B_out')
}

Signed-off-by: Cyril Hrubis <metan@ucw.cz>