preprocessor: allow passing mod file as string. Closes #1509

Usage: ./dynare_m $'<<mod file text>>'

The $’’ expands special characters. This is necessary because our setup for native matlab statements require that they end with a newline. In other words, the rest of the mod file can be sent on one line, but if there is a native matlab statement you must enter a `\n` after it.

NB: In this case, apostrophes must be escaped: ' becomes \'

e.g., to run tests/example1.mod:
./dynare_m  $'//Example 1 from Collard\'s guide to Dynare\nvar y, c, k, a, h, b;varexo e, u;verbatim;% I want these comments included in\n% example1.m 1999q1 1999y\n%\nvar = 1;\nend;parameters beta, rho, alpha, delta, theta, psi, tau;alpha = 0.36;rho   = 0.95;tau   = 0.025;beta  = 0.99;delta = 0.025;psi   = 0;theta = 2.95;phi   = 0.1;\nmodel;c*theta*h^(1+psi)=(1-alpha)*y;k = beta*(((exp(b)*c)/(exp(b(+1))*c(+1)))*(exp(b(+1))*alpha*y(+1)+(1-delta)*k));y = exp(a)*(k(-1)^alpha)*(h^(1-alpha));k = exp(b)*(y-c)+(1-delta)*k(-1);a = rho*a(-1)+tau*b(-1) + e;b = tau*a(-1)+rho*b(-1) + u;end;initval;y = 1.08068253095672;c = 0.80359242014163;h = 0.29175631001732;k = 11.08360443260358;a = 0;b = 0;e = 0;u = 0;end;shocks;var e; stderr 0.009;var u; stderr 0.009;var e, u = phi*0.009*0.009;end; stoch_simul;'