Proper alignment for diff rendering

[angely-dev]

The current diff rendering may be confusing:

hostname R1
interface FastEthernet0/0
 description LAN
 no ip address
 shutdown
 duplex auto
 speed auto
+ no shutdown
router bgp 64512
 neighbor 172.16.0.1 remote-as 100
 address-family ipv4
  neighbor 172.16.0.1 activate
  neighbor 172.16.0.1 prefix-list IN in
  neighbor 172.16.0.1 prefix-list OUT out
+  neighbor 172.16.0.1 allowas-in 1
+  network 192.168.1.0 mask 255.255.255.0
ip prefix-list IN seq 5 permit 192.168.2.0/24
ip prefix-list OUT seq 5 permit 192.168.1.0/24
+no ip domain lookup
+interface FastEthernet0/0.10
+ description LAN
+ encapsulation dot1Q 10
+ ip address 192.168.1.254 255.255.255.0

This could lead to think that some of the new items are nested in existing ones. Adding an extra space for non-plus lines will result in a proper alignment:

 hostname R1
 interface FastEthernet0/0
  description LAN
  no ip address
  shutdown
  duplex auto
  speed auto
+ no shutdown
 router bgp 64512
  neighbor 172.16.0.1 remote-as 100
  address-family ipv4
   neighbor 172.16.0.1 activate
   neighbor 172.16.0.1 prefix-list IN in
   neighbor 172.16.0.1 prefix-list OUT out
+  neighbor 172.16.0.1 allowas-in 1
+  network 192.168.1.0 mask 255.255.255.0
 ip prefix-list IN seq 5 permit 192.168.2.0/24
 ip prefix-list OUT seq 5 permit 192.168.1.0/24
+no ip domain lookup
+interface FastEthernet0/0.10
+ description LAN
+ encapsulation dot1Q 10
+ ip address 192.168.1.254 255.255.255.0

Actually, this is what GitHub does in its diff rendering (e.g., 03a24ab).

This enhancement requires to:

• Change the IncrementalDiff._to_str() method
• Update the unit tests
• Update the diff outputs on the documentation