Fix background palette bug

Made the background use pallette 0, and text use palette 1, so the background is shown correctly; that way, color 0 of the background image is displayed as expected.

Automatically convert images to 16 colors, if necessary

Merge pull request #78 from haroldo-ok/convert-image
This fixes #25

Allow the use of native `C` functions

• Implemented support for using .c, .h and .res on the project:
  • Before the transpilation, the required target dirs are automatically created, if they don't exist;
  • Before the transpilation, the contents of the base/ folder are copied to the src/ and res/ folders are copied to the project's corresponding folders; this means that the base libraries and base resources don't need to be added to the project during its creation;
  • Before the transpilation, if the project/ folder inside the project contains child src/ and/or res/ subfolders, those will be copied unmodified to the target src/ and res/ folders.
• Implemented new commands enable the use of native C functions on the project:
  • import: imports the given .h file into the generated code;
  • native: directly calls a C language function.

Fix image and sound related code to check if the referenced file exists

Merge pull request #72 from haroldo-ok/check-file-existance
This fixes #67

Implement automatic word wrapping for the text boxes

Merge pull request #70 from haroldo-ok/word-wrapping

Customize cursor and allow to select image layer.

• Flags were added to the image command, in order to allow choosing if the image will be drawn in the foreground or in the background layer;
• Implemented the cursor command, in order to allow customizing the cursor.

Implement `title`, `author` and `while` commands.

• title and author are used for populating the ROM header;
• while allows to keep repeating a block of code while a condition is true.

Create `window`, `flush` and `clear` commands.

• window: allows to change the location of the text window;
• flush: immediately displays the contents of the text buffer on the text window; it may or may not wait for a button press;
• clear: clears a layer/region of the screen.

Implement string interpolation.

Now it is possible to use print a numeric variable in the middle of a text line by using ${expression}.

Improvements on the command line tool

• It is now possible to choose which project to compile;
• The command line tool can now invoke the SGDK compiler to convert the generated .c and .res files into ROMs;
• The command line tool can now call the emulator after compiling;
• It is now possible to call a command line menu to compile the projects interactively.