don't auto-switch to newly picked up weapons if the player is firing at the time

[VoxelStorm Bug Submitter]

Submitted by slowriot from IRC (slowriot on borealis.voxelstorm.com)

[Riot]

adding saved setting to optionally switch weapons to a newly picked up one only if not firing at the time, resolves #84

→ <<cset 597fe525f62f>>

[Riot]

saving and loading weapon switch mode, re #84

→ <<cset 11477369fe29>>

[Riot]

add menu option to switch pickup modes, re #84

→ <<cset a16ca966d011>>