customizing filler character for `append`

I don't know whether to make this a bug or a feature request.
When using `task <#> append` to add additional information to an issue I tend to use a semicolon. Currently, `append` adds a space after the original string (or the string always contains a space at the end?), making this kind of separator ugly (space ; space).

An improvement would be:
- no character is added when appending
- there is a setting for what character(s) are added when appending.

This might be something to add to `prepend` as well.

----

* What command(s) did you run?
`task add "sth sth"
`task 1 append "; sthsth"`

* What did you expect to happen?
The end result should contain the text "sth sth; sthsth".

* What actually happened?
The text contains an additional space after where the first string ended "sth sth ; sthsth".

* Paste the output of the ```task diag``` command.
```tasksh> diag

tasksh 1.2.0
Platform: Linux

Compiler
Version: 6.3.0 20170618
Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
Compliance: C++11

Build Features
Built: Jul 2 2017 11:06:44
CMake: 3.7.2
libreadline: 7.0
Build type: None

Configuration
TASKRC:
TASKDATA:
PATH: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/lib/wsl/lib:/mnt/c/Program Files/Java/jdk11/bin:/mnt/c/Windows/system32:/mnt/c/Windows:/mnt/c/Windows/System32/Wbem:/mnt/c/Windows/System32/WindowsPowerShell/v1.0/:/mnt/c/Windows/System32/OpenSSH/:/mnt/c/Program Files/Adobe/Adobe PDF iFilter 11 for 64-bit platforms/bin:/mnt/c/Program Files (x86)/WinCVS_E121:/mnt/c/Program Files (x86)/Common Files/Pulse Secure/VC142.CRT/X64/:/mnt/c/Program Files (x86)/Common Files/Pulse Secure/VC142.CRT/X86/:/mnt/c/Program Files/Microsoft SQL Server/130/Tools/Binn/:/mnt/c/Users/sheidinger/AppData/Local/Microsoft/WindowsApps
Taskwarrior: /usr/bin/task 2.5.1
```

#### To request a feature...

* Clearly describe the feature.
* Clearly state the use case. We are only interested in use cases, do not waste time with implementation details or suggested syntax.
* Please see our notes on [How to request a feature](https://taskwarrior.org/docs/features.html)