Add arrow positioning option

[natenho]

Hello, I've just added this option to draw arrows at either different or both sides of the lines.

(all lines drawn from left to right)

The configuration is as simple as:

"red Marker end" = PEN (size=6 color="red" arrowsize=1 arrowposition="end");    
"red Marker start" = PEN (size=6 color="red" arrowsize=1 arrowposition="start");    
"red Marker both" = PEN (size=6 color="red" arrowsize=1 arrowposition="both");

I did it in a single commit because it was not a huge change, let me know whether it doesn't fit the contributing guidelines so I can split it up.

I was looking for a tool which mimics drawing features of Microsoft ZoomIt and gromit seems to be the right tool for that, but it lacks some drawing features that users like me would miss when migrating from Windows environment.

Another feature I want to work on is #67, for the same reason.

[natenho]

@bk138 thanks for your review! Actually I did a lot of changes since this PR that are in my fork, which include some cool features (based on MIcrosoft ZoomIt) like rectangles, lines, ellipses, and some other some fixes (like fixing some memory leaks I introduced). Currently I'm using my "version of" gromit-mpx in a daily basis and everything is fine!

I'll try to sync everything back here, but it will probably take a while. Regarding the whitespaces, well, vscode did that. lol. I'll get them fixed aswell.

[bk138]

@bk138 thanks for your review! Actually I did a lot of changes since this PR that are in my fork, which include some cool features (based on MIcrosoft ZoomIt) like rectangles, lines, ellipses, and some other some fixes (like fixing some memory leaks I introduced). Currently I'm using my "version of" gromit-mpx in a daily basis and everything is fine!

I'll try to sync everything back here, but it will probably take a while. Regarding the whitespaces, well, vscode did that. lol. I'll get them fixed aswell.

Super nice - Looking forward to them!

[bk138]

Closing in favour of #195.