Adds support to user defined line list

IDL Synplot has an argument called `linlist` that allows the user to set
a line list. I have added support for this.

Also, I have to do a workaround to avoid that Synplot.pro erases the
original line list (fort.19) file.
Releases version 0.2.3
Start development for 0.3

I have created a milestone for that:

https://github.com/gabraganca/S4/issues/milestones
Merge branch 'development' into feature-fitting