New collect feature : local mountpoints

Hi !

I've made a whole new gather function to collect local FS usage metrics.
Supported FS can be modified easily and is (for the moment) : vfat, ext2, ext3, ext4, btrfs, zfs & ntfs-3g

```bash
> ./sysinfo_influxdb -c mounts

#0: mylaptop.mounts
| mountpoint	| fstype	| disk	| free	| total	| usage	|
| /	| ext4	| /dev/sda2	| 117969395712	| 125855354880	| 0.063	| 
| /home	| ext4	| /dev/sda6	| 50834329600	| 242004779008	| 0.790	| 
| /boot	| ext4	| /dev/sda1	| 329293824	| 486123520	| 0.323	| 
```
Maybe the "disk" column is not absolutely necessary (as fstype) but it can be removed very fast if you want it (it could reduce the amount of data sent to influx).
I've added mounts options to the defaults options, but it can be deleted and then become and full optionnal func... Just tell me what you prefer !