* add make_printf_args and make_wprintf_args to printf.h
add minimal test for make_printf_args and make_wprintf_args to printf-test.cc

renamed the old printf_context template to basic_printf_context_t. the original wish was to rename it basic_printf_context, but that clashed with the name of the inner typedef. this style matches the format_context_t struct.

- #926
- The version of android build results follows latest release(5.2.1)
- Comment for Gradle and its plugin for android
 - `net.fmtlib` package name in the manifest

Recovered Gradle build with latest release (ver. 4.10.2)

TravisCI
- Changed several apt configurations & build steps for Android

AndroidManifest:
- package name: fmt -> org.fmtlib

Gradle:
- Plugin version 3.1.3. Requires Gradle 4.4+
- Removed `x86` (duplicated with `x86_64`)
- `-std=c++17`. Latest NDK(clang7) should support this
- Missing `repositories` configuration

For some reason, the `repositories` config affects to the error:
"Could not find com.android.tools.lint:lint-gradle:26.1.3."

Recovery of Gradle build. 1 of N
Modified several apt configurationss and order of the build

This reverts commit 07200f44 because
the latter doesn't work with virtualenv correctly.

checks for clashes with other function overloads in fmt with templated parameters on the first or second position in the parameter list

std::back_insert_iterators model the OutputIterator concept but differ considerably in their traits and behavior. In particular the former made compilation to fail when format_to_n is given a back_inserter as first argument. The emulation of an OutputIterator is not perfect due to the behavioural differences of back_insert_iterators (e.g. assignment always implies increment) but good enough to be used within fmt's machinery.