Apply GitHub style to exporter matrix

[yennster]

Description

This PR changes the table output produced by mbed export -S from this...

To this...

Which allows simple copy/paste in github style table markdown (similar to PR #7720), for example...

Platform	cces	cmake_gcc_arm	codeblocks	e2studio	eclipse_armc5	eclipse_gcc_arm	eclipse_iar	embitz	gnuarmeclipse	iar	make_armc5	make_armc6	make_gcc_arm	make_iar	mcuxpresso	netbeans	qtcreator	sw4stm32	uvision5	uvision6	vscode_armc5	vscode_gcc_arm	vscode_iar
ARCH_BLE	-	x	-	x	x	x	-	-	x	-	x	-	x	-	-	x	x	-	x	-	x	x	-
ARCH_BLE_BOOT	-	x	-	x	x	x	-	-	x	-	x	-	x	-	-	x	x	-	x	-	x	x	-
ARCH_BLE_OTA	-	x	-	x	x	x	-	-	x	-	x	-	x	-	-	x	x	-	x	-	x	x	-

And it just looks cleaner and is easier to read.

I also added another "\n" blank space in between the end of the table and the table information (total IDEs, platforms, etc.)

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[X] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[cmonr]

@yennster Could you rebase your PR?
I don't know why, but Travis CI is being weird and not updating itself correctly.

[cmonr]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts