Use system installed hoedown library on "Unix"

Link to the system-provided hoedown library under /usr/lib on unix-like
systems.

Closes #157
Activate hoedown feature on travis CI
Update 3rdparty/hoedown
Fix path to hoedown headers on unix #157

We now expect the hoedown headers to be in path /usr/include/hoedown on
unix-like systems.
Fix install target for the application on unix

The application executable is now installed to /usr/bin by default
on unix-like systems. The installation prefix can be changed through
the qmake variable PREFIX.

Closes #154
Use unix-specific logic to find dictionaries

Search several distribution specific system paths under /usr to find
all dictionaries.

Closes #153
Use QStringLiteral for dictionary paths
Fix saving of user snippets on linux (#152)

Add wrapper class around QStandardPaths::DataLocation that ensures
that the path returned by writeableLocation() exists.
Bump version to v0.10.1
Adapt rpm spec file to changes
Update README.md for v0.10.1
Merge branch 'hotfix/v0.10.1'