This function plots the mean and standard deviation of a set of data filling the space between the positive and negative mean error using a semi-transparent background, completely customizable.

Input parameters:
- data: Data matrix, with rows corresponding to observations and columns to samples.
- options: (Optional) Struct that contains the customized params.
* options.handle: Figure handle to plot the result.
* options.color_area: RGB color of the filled area.
* options.color_line: RGB color of the mean line.
* options.alpha: Alpha value for transparency.
* options.line_width: Mean line width.
* options.x_axis: X time vector.
* options.error: Type of error to plot.
if 'std', one standard deviation;
if 'sem', standard error mean;
if 'var', one variance;
if 'c95', 95% confidence interval.
Example of use:
data = repmat(sin(1:0.01:2*pi),100,1);
data = data + randn(size(data));
plot_areaerrorbar(data);