Make date parsing context aware

It is Tuesday (and I have already some bookings today) and I run `timew summary monday - today` assuming that the output contains Monday and Tuesday, but it's only showing the bookings on Monday.

I think it's easy to reproduce for you guys, so I'm not attaching anything here besides the diagnostics:

```
$ timew diagnostics

timew 1.2.0
Platform: Darwin

Compiler:
Version: 4.2.1 Compatible Apple LLVM 11.0.0 (clang-1100.0.33.12)
Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
Compliance: C++11

Build Features
Built: Nov 29 2019 16:17:45
CMake: 3.15.5
Build type: Release

Configuration
TIMEWARRIORDB: -
Cfg: /Users/myname/.timewarrior/timewarrior.cfg (-rw- 0 bytes)
Database: /Users/myname/.timewarrior (drwx 160 bytes)
Color theme: Built-in default

Extensions
Location: /Users/myname/.timewarrior/extensions (drwx 64 bytes)
(None)
```